I expect a curve (Kojima said within 10 minutes a curve comes, must be during the opening movie...heh) much like MGS2. I'm thinking what we've been shown is only a small part of the game, a flashback if you will (akin to the ship part in MGS2). Then it will come back into modern times. This is the final game of the trilogy after all...I can't believe he'd leave the series that wide open with questions, but what do I know?
That Kojima quote is weird (I read it before too). The demo seems to contain at least the first 30 or so minutes of the game and there really isn't a "curve" unless you count the already widely known fact that the game takes place in the early 60's and you're playing what appears to be proto-Snake Big Boss.
I've seen the "modern times" switch mentioned pretty often but I think Kojima has gone on record that you won't be switching characters a la MGS2 so that would make that would make a modern times transfer a bit more difficult.
As to outstanding questions, many could conceivably be answered during the prequel timeframe. I woldn't be surprised to see any outliers covered in Metal Gear Acid which evidently takes place post MGS2.
Then again, maybe the demo is a bit of a ruse and the "switch" won't appear until the final version. Its the same demo they had on the floor at E3 I believe.
Re: PoP delay- my guess is they are trying to get clear of the GTA, NFSU2, Halo 2, MGS3, MP2 blitz that will be late October to mid November. Things are comparitively quiet in early December with GT4 and KOTOR2 the only major releases I can think of.