The part about "how does Fate handle wild-ass Declarations" has been answered, but regarding the questions about how locked-into-stone Settings are:
"Is it possible to reconcile a "closed/fixed world" where the setting is as it is predefined by the GM (or other) and Fate system?" Of course. There's no conflict. Fate gives you a way to get on the same page about the world before play starts. If you aren't doing "game creation" like in the second chapter of Fate Core, you still need to have a group who's on the same page about the game and its setting and world. Personally, I recommend a "Session Zero" for any game, even if it's using published setting and adventure material, like a Fate World. If it's a one-shot, the GM does have a lot more responsibility as well as permission to establish things.
"Can the GM (or the player responsible for the setting) unilaterally deny actions that have no place in that specific world?" Of course (see rule cited in doppelgreener's answer). The GM should be prudent - if it's unilateral and it's against unanimous, vocal support from all players present, that might not be the right choice.
"must it be a majority vote or should the table arrive to a consensus?" To Declare a Detail? Probably not, but Maybe. Especially if it's not a Detail but it's actually a Setting Element. This should have happened before roleplay started. During Session Zero, when you're all following the Game Creation and Character Creation chapters, or else reviewing the gamemaster's published or otherwise pre-defined material. So that it isn't a disruption in the middle of a session.
Here's the thing: The Declare a Story Detail rule should be used for points of plot, not for establishing world/setting facts.
It's the difference between spending a Fate point on "Since my character is retired from the Academy, it makes sense that the Academician who just got called as an expert witness has cited my work. So, can we say that my lawyer can argue that she's not impartial by using that fact against her and discredit her testimony? offers Fate point" and "holds out token Here's my Fate point, let's say that all I have to do to escape these orbital patrollers is put my starship into Hyperspace... Yeah, we really need that in this universe anyway..."