O.K. first of all let's deal with the term 'romance' within the context of this game. It falls in to two categories. A relationship and a one night stand.
In the base game (i.e. no Expansion), under the category of 'Relationship' we have Triss and Yennefer. Under the category of one night stand we have Keira, Sasha, Jutta and the whores.
So where does Shani fit in this when we add HoS in to the equation? Because Shani says "I need to sort this through in my mind" it's clear that it's presented as a potential relationship, not as a one night stand. If you're simply enjoying carnal delights there isn't anything to sort through in your head. However, in the other two relationship romances (Triss and Yen) the player can choose to settle down with them or not.
This choice is removed from the player in the Shani romance despite it being presented as a relationship. As a consequence of this lack of choice the player is funneled down a linear story with Shani, a girl who wants a relationship to define her life rather than sleep with some guy she happens to like a lot each time he wanders in through her door.
This is what's out of step with the other romances. It's got nothing to do with the character, the story or what you believe you read between the lines, it's the removal of player choice that is inconsistent with the other relationship-based romances and as a result, Shani's romance ends up falling somewhere between the two categories and succeeding at neither. It would not have taken much work to either make it clear this was a one off or to allow those people who went to bed with Shani at the end of The Witcher and woke up with Triss in The Witcher 2, to have re-kindled their old flame. The games already diverge from the books by allowing Geralt to settle down with Triss, why not allow players to settle down with Shani? It only needed an end-game card and a one-liner from Dandelion. Either way, the fact that it does neither is what makes it inconsistent with the other two.
I never said that Shani went into the wedding thinking that this was going to be a one night stand. She posted a note on one of the bulletin boards near Oxenfurt which said that she was looking for a gentleman to accompany her to a friend's wedding, and that if he behaved himself, she might let that person court her properly. Fortuitously enough Geralt shows up, someone who, depending on player choice, might have had previous romantic dealings with her. So yes, I agree, she was not looking for a one night stand, she is potentially looking for a committed relationship and a man that she can settle down with. What I don't agree with is the assumption that just because she and Geralt might have once been an item, and because they decided to rekindle that romance at the wedding, that it is now incumbent upon Shani to make something permanent out of it. She said "I need to sort this through in my mind," as you yourself pointed out, which is obviously an indication that she has mixed feelings about what just happened. There are a myriad of different reasons why she might not want to marry Geralt, despite their previous romantic entanglements: he's infertile and incapable of having children with Shani, his lifespan is significantly longer than hers, he hasn't proven that he is capable of commitment and subordinating his own interests to that of another person (yes, Geralt eventually does just that with Yennefer or Triss, but Shani has no way of peering into those other potential realities.
You keep mentioning choice, how it is removed from the player despite the developers presenting this as a potential romance. But what about Shani's choice? You're basically saying that relationships in the world of the Witcher break down into two groups, flings and marriages, that all characters must fit neatly into these two categories, and if a character is not a fling then Geralt must have the option of marrying them. That's pretty twisted logic and way oversimplifies how relationships work.
It has everything to do with character development, story, and reading between the lines. Video game romances get a bad rap precisely because realism is relegated to the back-burner in order to accommodate player choice. In some ways this reminds me of the debates over npc sexuality over on the BSN. You would fall into the camp that says, "It only takes a few extra lines of code to make an npc gay or straight. Why not make everyone happy and create exclusively bisexual characters?" I would retort, as I seem to be doing in your case as well, that relationships do not work that way in the real world.
Anyways, I don't think we are going to arrive at a consensus. It's pretty clear that you conceptualize video game romances in a completely different way than I do. You seem to prioritize player agency, whereas I believe that developers should try to mimic the complexities of real life relationships, even at the expense of player choice.