I'd like to take issue with some of the things the OP says and assumes about narratives, what constitutes an "optimal" narrative, and what the game's narrative constitutes. I'd also like to interpret the ending, particularly Ciri's taking on the White Frost, as a positive aspect of the game's narrative and not a deus ex machina as the OP portrays it.
But first, assumptions!
There is a certain, "optimal" pattern for how to pace a good and gripping story in writing theory.
Really? I'd like to know what this "optimal" pattern for pacing is. I've never heard of this pattern before. If it's so optimal, then why isn't it used in every narrative? How does it manage to work for every single story one can tell? Does this mean narratives that are constructed with a different pattern in mind, such as
The Life and Opinions of Tristram Shandy, Gentleman, are inherently flawed? Is
Ulysses a flawed narrative because... well, everything about it? If every narrative used this "optimal" pace, wouldn't it be extremely dull?
Let's start with the smaller problems, the inconsistent pacing structure with main moments in the game that are more engaging, emotional and gripping than the final battle.
There is a thing, in storytelling, called a climax. It usually happens around the 1/2 or 2/3 point of a story, and it's the moment that defines the story. In the OP's analogue, Star Wars, both within its movie and in the broader context of the original trilogy, that moment is when Darth Vader tells Luke that he is Luke's father. It's a moment that is more engaging, emotional, and gripping than any other part of the original trilogy, the battle on Endor included. In The Witcher 3, the climax of the game is around the 1/2 or 2/3 point of a story, from Geralt's reunion with Ciri to the Battle of Kaer Morhen. There is literally nothing wrong with that segment of the game being the best and most memorable part of it - in fact, from a storytelling perspective, it's encouraging that that is the game's high point. Why should the final battle with the Wild Hunt override that part of the story?
In theory, there are two types of possible arch-type villains, the narrative villain and hte force-of-nature villain. While they have different traits and motives, they share one definite element: both can't just be "evil" for the sake of being the evil force in the game.
These are unclear terms simply through their names. What are these two types of villains? How do I know you're not making stuff up for the sake of making stuff up? And why can't there be an archetypical villain-for-the-sake-of-having-a-villain? The older James Bond movies are filled with such villains, with unexplored motivations, such as Oddjob, and yet they worked well in their role as villains.
Especially everything sexual is rarely a topic although it's actually a quite important theme for Ciri since "everyone wants to abuse here sexually".
... What?
No, seriously, where is that quote even from? And by your logic, any story that involves Ciri should heavily involve people attempting to abuse her sexually - that is not remotely sensible.
Moving on from assumptions, I'd like to present a perspective of the game's story, especially the last third of it, that is positive. The Witcher 3 is explicitly about Geralt's tale - Dandelion narrates it, as he narrates the first two games, and thus the events in the game are presented entirely from Geralt's perspective, with the exception of the Ciri parts. We can break down the parts of the game's story where Ciri is playable into two sections: flashbacks and supporting element. The flashback scenes occur exclusively before Geralt and Ciri reunite, and they are always placed in the context of someone telling Geralt about those events (The Bloody Baron narrating the fight with the basilisk, the Crones narrating her escape from the swamp, Whoreson Junior narrating Ciri's rescue of Dudu, etc.). They likely exist for the player to build a connection with Ciri, to avoid having side characters narrate endlessly about her actions without actually making her visible, to make her less of an object or a goal and more of an actual person. It also illustrates her growing power. The second type of Ciri section, the supporting elements, occur exclusively after Geralt and Ciri reunite, and they take place during the two major battles Geralt is involved in; these scenes are basically her helping Geralt out during those battles. The important thing is that those scenes serve to supplement Geralt's story, not tell a separate one about Ciri.
That's why the ending is as it is. The player's objective, as Geralt, is to defeat the Wild Hunt and protect Ciri from them. He accomplishes this in the final part of the game, before thinking that Avallac'h has kidnapped Ciri and rushing after him, only to be informed otherwise by Ciri. Ciri's defeating the White Frost is an altogether different quest from Geralt's quest, and the fact that it isn't shown is actually a good thing. It fits with the entire theme of the last third of the game - Geralt, as a father figure to Ciri, is supposed to prepare her for her own, separate adventures; and he is preparing himself for when he has to let go of Ciri and let her have her own story. In Geralt's story, it doesn't matter how Ciri defeated the White Frost: what matters is that she is either successful in doing so, meaning Geralt has succeeded in enabling her confidence, or she is unsuccessful, and Geralt has failed in enabling her confidence.
Let me establish my own analogy here: Ciri is like Bruce Lee, and Geralt is like Ip Man. They were both brilliant martial artists in their own right, and Ip Man was one of Bruce Lee's teachers. But if you were to tell a story about Ip Man, you wouldn't talk about Bruce Lee's role in
Enter the Dragon. You wouldn't define Ip Man by the accomplishments of his protege. You would talk about how Ip Man taught and mentored Bruce Lee, or how Ip Man enabled Bruce Lee to become the top martial artist that he was. That's assuming you tell a story about Ip Man that involves Bruce Lee, of course; you could easily talk about Ip Man's accomplishments before he met Bruce Lee. This is the exact same way the story works with Geralt and Ciri: Ciri is Geralt's protege, and Geralt mentors and enables her to do great things, but he doesn't go out with her to defeat the White Frost. That's Ciri's story, her legend if you will; while Geralt's legend is defeating Eredin and the Wild Hunt. That's what the game focuses on. It also fits thematically: the overall moral of the story, of the last third of the game, is about allowing, and enabling, your children be better than you, and to not helicopter them on everything. Let's run down the list of major moral choices:
1. Vesemir's death and consoling Ciri: you have a choice between telling Ciri that she may be literally incapable of doing something, or giving her a fun break from her studies. The second choice enables Ciri to become better, and is thus considered the "good" choice.
2. Accepting or refusing the emperor's reward for finding Ciri: contrary to OP's post, the choice isn't whether to go to Velen or to go to the emperor first, but whether to accept or refuse the emperor's coin if you do go. Accepting his coin in front of Ciri tells her you're out for personal gain, while denying it affirms that you brought her to the emperor simply for Ciri's sake, so she can hear him out and make her own decision, hence, it is considered the "good" choice.
3. Interceding on Ciri's behalf to the Lodge or letting her go on her own: this is, again, about helicoptering a child. The "good" choice is to encourage her to handle the Lodge by herself, while the "bad" is to go yourself, speak for her, and basically shelter her. The bad choice tells your child that you think them incapable of fighting their own battles.
4. Destroying Avallach's lab or holding Ciri back: while not a nice or a professional thing to do, and while it's not a "witcherly" thing to do, it's not about what Geralt decides, it's about what Ciri decides. Again, supporting her is the "good" option, as it should be. It's about breaking free of the chains of Avallach's tutorship of Ciri, that Ciri can do whatever she damn pleases.
5. Visiting Skjall's grave or not visiting Skjall's grave: again, this is about what Ciri wants, and not helicoptering her. If she wants closure, a good parent will damn well help her get some closure. I'm not even sure this one is up for debate.
All of them are tied to the same theme of enabling Ciri, of allowing her the use of her skill and power. The OP speaks of a flawed chain of causality, where our choices feel insignificant compared to the consequences. Nevermind that the OP praises Chrono Trigger immediately beforehand for having
the exact same type of chain of causality, the point is that the choices the player makes in The Witcher 3
are tied into their consequences. The fate of the North is dependent on whether Geralt aided in the attack on Radovid and whether he supported Djikstra or Roche afterwards; this effects Emhyr's fate, since Emhyr either wins or loses the war depending on who's leading the North. That's a solid chain of causality. Whether mages or nonhumans are persecuted in Novigrad - that's tied directly to whether Geralt helps the mages escape the city. If the OP is wondering about the endings themselves, about enabling Ciri through small actions, then the explanation is simple: the consequences are based on Ciri's choices, which are based on Geralt's choices. Either Ciri doesn't have the confidence to fight the White Frost and she dies, leaving Geralt in shambles; Ciri defeats the White Frost and met with Emhyr, where she makes the choice to become the queen of Nilfgaard; or Ciri defeats the White Frost, did not meet with Emhyr, and becomes a witcher like her adopted father. Your choices impact Ciri's choices, and you learn the consequences of those choices as well. If that's too disappointing for the OP, well, that's the OP's problem, not one with the game's narrative.
A few other notes, since I've been rambling on and on:
The problem here is: Eredin is neither one or the other. Throughout the whole game he stays blank, like the big evil in the background that is just needed to have any kind of goal.
Did you miss the entire explanation of: Eredin usurped the previous ruler of the Aen Elle to take power, discovers his people are about to be consumed by the White Frost, and wants Ciri and her power so he can save his people? Or are you upset that Eredin didn't sit down for 5 minutes with Geralt and monologue his motivations to the audience? His motives and personality are quite clear, it's just that they're established through secondary sources like Ge'els and Avallac'h.
Besides from the obvious sorceresses, the other sorceresses feel underdeveloped as well, with the one exception of Keira Metz. But both Margarita and Fringilla feel rushed and pointless. They are both introduced rightly before the end and they don't have much purpose besides being used as staffage. It doesn't help her of course that the sorceresses and the lodge aren't even mentioned in the epilogue...
This ties directly into the issue of perspective. We don't know much about the sorceresses and their role in the end because Geralt doesn't hang around them, he's busy preparing for the battle in other ways. If Yen was a secondary protagonist, we would almost definitely see more, but again, this is Geralt's story set at a particular point in time. His experience with these sorceresses comes prior to this point, and for some members, it's covered fairly well in the second game.
Especially the relationship between Ciri and Yennfer is extremely underdeveloped.
Perspective again. Why would Geralt tell Dandelion about moments that Ciri and Yen had by themselves? That's part of Ciri's story, not his.
Most of the videos in the epilogues are about political outcomes (even thought the game should be about Geralt's personal quests) and even those are lacking. Take for example the events on Skellige Islands. Right before the end there is both an upcoming civil war and a Nilfgaardian invasion established in the narrative. No matter which epilogue you get, no word on these events in the epilogue videos which seems weird. Why establishing a civil war right before the end if you just ignore it in the epilogue?
The game is about Geralt's personal quest, and that's what the ending is for. The epilogues are supposed to tie up loose ends relating to the overall picture around the story, hence the focus on politics. And events on the Skellige Isles are explained? I assumed the "upcoming civil war" was led by Madman Lugos, who Geralt and Ermion kill during the game, and the "Nilfgaardian invasion" isn't an invasion, it's just Emhyr dedicating his resources to the fight with the Wild Hunt. Even if he was invading, he surely wouldn't have the forces to do so after that battle. The epilogues then go on to explain what happens in Skellige in the future, depending on the ruler: Cerys promotes peace, Hjalmar raids Nilfgaard from time to time, or Bran's kid becomes the first in a line of hereditary monarchs and the Isle's influence wanes.
The one person who seemed to be weak only one moment before just saves the world in the next without explaining how the suddenly had the power to do so and how she did it in detail.
Ciri is anything BUT weak throughout the entirety of the game. Don't confuse everyone's desire to protect her, and their locking her down, as a sign that she is weak. Her power is established very clearly at the Battle of Kaer Morhen. I don't understand how anyone could think she was weak, when pretty much the entire point of the game is that she's extremely powerful.
And as for detail about Ciri's capabilities, this goes into an argument about fantasy in general, and magic in fantasy, that I don't want to get into, but can be summed up fairly well by this 9 minute video:
So, in conclusion, I think the last third of the game, its Act 3, isn't "a bad, hot mess". I think it's respecting of the story's narrative structure, the story's objective, and it doesn't break from either the story's structure or its theme simply to satisfy the player. I don't believe the ending is perfect. I think the relationships between Geralt and Yen or Geralt and Triss (and particularly Triss) aren't fleshed out enough and aren't handled well near the end. Perhaps that would have reminded players of who the story is really about here. But I don't think the ending is fundamentally flawed; quite the opposite, it's an interesting ending. I feel like book readers are disappointed by a lack of detail that I feel shouldn't even attempt to be replicated in a video game or a movie, if only because it robs the audience of that detail. And I feel that pure video game players are disappointed by not being able to see Ciri take on the White Frost, even though that's sort of the point. I'm not inventing a cop-out for CDPR here, I'm simply presenting what I feel to be a plausible, and well handled, interpretation of what the story of The Witcher 3 is really about. It's how I interpreted the ending as I was playing through it.