I agree with most of what you've said and I think it is indeed at least half-decently possible and realistic with really well done writing and story.
But as you've correctly pointed out there must happen something at certain points in the game that is not arbitrary if you want to maintain at least a certain amount of tension. The problem I have with this whole topic is how CDPR advertises its approach: " Do anything you want, anytime you want!" Either that's a lie and the game does actually have certain times in which your gaming experience is directed and at least partially linear or it's indeed true and that's actually the interesting point because it could mean different things, depending on how it is implemented. That either means that the overall plot arc evolves around you on its own and you might participate or not (and bear the respective consequences) or it's the traditional tension and plot-pacing breaking experience we are used from other open world RPGs like Skyrim in which the world is not made to be immersive and believable but to be the gamey sandbox just created exclusively for the player. I think the former possiblity is probably the most interesting one (it's pretty much a continuation of what I've described in my latest post where I critiziced the plot of Dragon Age Origins) but also the hardest one which has - at least in my experience - never been done in an RPG because it includes so many variables and possible outcomes. While I would like to play such a game I don't think it's realistic on such a big scale. So I think it's maybe a cross-over between this approaches and the advertizing (the sentence is a direct quote from W3's shop page on Steam) is just exaggerated or overstated because the player certainly will be directed and "guided" somehow. The problem that remains is that while offering too much freedom you also give players the opportunity to "destroy their own experience". That may sound weird in the first moment and you might say "that's not CDPR's issue" but I don't think it's that easy. By giving the player too much freedom you also give the player the opportunity to follow his own weaknesses (or just his curiosity) while not knowing the consequences for his overall experience. Such things are dangereous because they easily break the immersion and what else do you want to achieve in a narrative game if not immersion, engagement and presence? It's not easy for a developer to find the right balance here and giving the player "full freedom to do anything in every moment" is like opening a can of worms imo...
I'm really considering opening a new thread about open world design or general design of narrative games. It's definitely an interesting subject when you look at it from a game designer's perspective and not only a player's one...
Ah yeah, the way they advertise it is indeed contrary not just to the ultimate reality but also to what we want/hope/think it'd be the best. As you I dont really take it as literal, I mean when people ask them "woah !! is it really open world? or not so open world", its natural that they just cut down to the chase in order to be clear and say "yeah you can do anything you want anytime",they're not gonna get into the super specific mechanics and solutions, but its nevertheless a concern that might turn out to be real.
I agree that the second possibility of the story building around your own take is appropriate, at least from a design on paper view, but also the hardest one. I havent really thought about this too much since I'd take me some extra time, but I think that they will try to make the story quests and events "be available" depending on where you go and what side quests consequences you trigger, and that would just give you the freedom of a perspective and main story choices, while also allowing you to "abandon" a main story part, and yet follow it indirectly by moving unto some other place or side quest.
If I remember correctly, each big/medium place, and their important chars and such matter in the context of the main story, and its a big main story with wild hunt invasions and war that by logic simply somehow end up relating those places and characters, because they are transcendental events. Now why do I think the game is like this? because the battle of developer direction vs player freedom, in general, ends up in that the best compromise between the two, is to give the player freedom but control his/her psychology and emotions so much that they still end up going for the "developer's path", but while retaining their freedom, its pretty much the ultimate solution really, the players end up doing what you want, but not only they still feel free, but also this freedom makes them think they are responsible for the great story you "secretly"
made them have, made them have with suggestions, clues, red herrings, etc etc. So yes, I think that the whole thing of "you can rebel against the mains story and go do something else" will naturally end up putting you once again into the main story, only in a different unexpected way, perhaps another section, another perspective, etc.
TW3's main thing as stated by developers is the main story, and so I believe you will never be able to escape it unless you go to extremes (standing in the grass doing nothing or killing randomly generated bandits), you might refuse to do a particular quest, thats fine, the consequences of that will catch you later and in fact be related to the things you are using to escape the main story, such as what first look like totally disconnected side quests, but in reality are not.
Another thing they said is that you sort of find different pieces of the main story and then it starts putting itself together up until its very well formed and leads to the end, this fits my opinion here. In a game where consequences have real impact and change the world around you, the ultimate problematic player freedom, is simply in the beginning of the game, once the player passes that part, and initiates a "world reaction", the game can now have an argument to throw things against you because of what you did before, and those things get chained more and more as you play, in essence, the player might have freedom, but because of the consequences they themselves slowly lock their own freedom in the world, they act and act and limitations(story consequences and logical reactions) start coming more and more, until you have a much more streamlined set of ways to follow the main story near the end.
At this point I wouldnt be surprised if the whole northern kingdoms had succumbed to Ithlinne's Prophecy already, and become a snowy wasteland where you could hardly have dumb sidequests about some poor monster to follow.
As you said, the variables are truly many, however, combine what I said, with The Witcher franchise's "taste" for "destiny" and impotence of the anti-hero, like not being able to stop the war at the end of witcher 2, or not being able to stop Javed from escaping in the swamps of TW1.
A player might have freedom to start the story, and travel through its early events, yet quickly find
Gates of the World (another thing they said to be using), that pretty much unavoidably lead into the next big "step" of the main story, or abstract chapter if you will, thus like I said, limiting much of the freedom you used to have, perhaps according to some of your early actions.
The problem with player's freedom and them actually falling into the story CDPR want imo is a serious one, but if you build the story correctly to address it, it can be relatively solved. The main threat is the beginning of the adventure, the "blank slate", after that players will make choices and CDPR can respond with appropriate limitations (story reactions) that reduce freedom.
Now what would happen if players eliminate content/places accessibility before they could see it? well there are exceptions to solve that, but its a whole other wall of text. I suspect CDPR will simply employ plots and quests that make sure to take you around many places so you can enjoy the world before the world gates start to slowly forbid sections and quests.
There was a thread about the open world and how we thought they should/were to implement it, search for it and bring it back if you want.