GM: "Throndor, what lies to the west of this forest?"
Throndor: "A river, beyond which is the Elvish realm of Lothwithien."
Player of Elf PC local to the area scratches head and wonders how and why she's never heard of this place before now...
An extreme example perhaps, but done to prove a point: if something's authored to be there now it means that thing has always been there, and as soon as prior knowledge of its presence might have changed past events in the fiction (here, the Elf might have invited the PCs to go up the other side of the river [also previously unknown-of!] and visit Lothwithien instead of going through the forest they're in, had she known about it) then IMO things have become degenerate.
Put this all on a player-visible map ahead of time and stuff like this just can't* happen.
* - not without a lot of work and some DM errors, anyway.
Well, your first point doesn't seem too substantive to me, that is the player certainly doesn't know all the things that the character knows, so it should be no surprise to the elf's player that she is not cognizant of this fact.
Point 2 is harder to judge, as it is highly situational and also admits of a lot of interpretation. I mean, OK, Its POSSIBLE that if the player knew about this fact earlier in play they might have made different action declarations. However we don't KNOW that for a fact. Characters have many reasons for things, and in fact we can always invent some, particularly in a Story Now type scenario where most things are up for grabs. For example Lothwithien could be a secret kingdom which the elf is sworn not to disclose. Surely in a DW game this kind of explanation can be easily deployed, and surely various others also spring to mind.
Now, I won't try to argue that NOTHING could ever be declared that would come across as inconsistent with the established fiction. No doubt there is a range there as well. If the PCs took the road from Bogwood to Warden and then some later revelation of lore tried to put a giant impassable swamp there, then the GM should be calling that out, like maybe "Oh, you guys took a road through that swamp, right?" or maybe the fact is it was described as primeval forest, so the swamp is just out, the GM will need to invoke consistency with established facts (I don't recall there being any significant discussion of this in the DW rules, but I believe ultimate veto authority on fiction is at least implied to be vested in the table as a whole). Obviously it could be even stronger than that, too.
However, I think most cases of players authoring fiction in this way work pretty well. Remember, it isn't in ANYONE's interests for the fiction to be a muck. Its not like the players are little fiction destroying beavers. Nor does the GM know more about the setting than the players. He has IDEAS perhaps, but IMHO his only recourse if, say, a player spouts an answer that negates a danger of a front (basically an encounter grouping) that the GM already worked up would be to say "I'd like to contradict that answer, could Feldwen be mistaken and the Great River turns north before that point?" or even just call for a convo with the players out of character and hash out his issue with it. When it comes right down to it though, the players could simply come back with "nope, you haven't established whatever it is yet, so if we negated its existence that's on you." Obviously this is different from pre-authored games where the GM's fiction must prevail.