wedgeski
Adventurer
From what I've seen, my next D&D campaign will almost certainly be DDN, but really the only mystery that realistically needs to be answered by anyone right now is the adventure support, which is important, at least to me, not because it'll be a deciding factor in my purchases but because it will inform the amount of work I'm going to have to do.WotC has a horrible track record with these things, so an "all-in" consumer might want to know the answers before buying in.
I'm sure Mearls or someone could tell us right now how many adventures are in the pipeline for the first, say, three years of the game, and that information might even be forthcoming prior to release.
As for the other questions: the edition will last as long as it makes money; everyone's ideas of "core" are different so, absolutely, some players are going to have to wait for things they'd prefer to see in the core books; and a 3.5-type re-work may or may not occur depending on how the game is selling, but it's likely they'll plan for it in case it's needed and we should assume it will happen at some point.
Honestly, if WotC came out right now and said "There will be no mid-stream update!", I'd have a hard time believing it anyway and all it would take is a change in authority over there and the decision could be reversed without the batting of so much as a single eyelid. The community would piss about broken promises and would just be ignored.