my thought on the subject.
(just my hunch, not powered by facts or manga proofs).

the case of learnning the sealing jutsu is really a stupid argument, if you say that the 3rd couldn't have learned it before the 4th died, you might as well say that the 4th couldn't have learned it without his master dying to explain the jutsu...

as for the sealing target, I'm guessing that the 3rd could seal orochimaru into himself since both were in the same state (verge of dying, and pretty damn mortal, despite thier best efforts), however, in the state of the 4th, he probably needed someone with enough 'lifeforce' to keep back the demon. also, if you consider the fact no one knows what will happen to the kyuubi once naruto dies, perhaps the 4th was afraid that if he seals the demon inside him and then die right away, the demon will be released, meanning that nothing was achieved. so maybe the 4th decided to seal the demon into a child to buy off some time to the village to rebuild itself and produce a new ninja generation.

Kagebunshin seal - I really don't get this thing, you guys actually think you can trick dear ol' death to take a shadow clone's soul? this isn't a shitty freshman p2p program, there aren't any loopholes and code errors in the deal, you can't stand the heat of putting away your soul? don't stand in the battle kitchen.
and by the way, since the two dead hokages were already dead at the time, they probably came in the bargin deal of wasting oro away, so the 3rds' bunshins were just thier as a medium to channel the jutsu energy.
oh, and his body bag? probably didn't appear since Kishi doesn't want us to know how the 4th looks like. or maybe it turned out that the dosu corpse he had wasn't fit to be a ahokage contanier (it was rotting in the sun for a month after being smashed by Gaara, I can't even see how it maintened human form).

morally speaking, Naruto can't be the 4th incarniate. if he was then his acsent (spelling?) to the hokage seat was assured, which would render the storyline pointless (as in "Naruto wasn't a loser who worked hard to be the hokage, he was fuckin' born after having been declated the strongest hokage ever"). but I'm counting on Kishimonto to mess this one up as well.
Maybe the 4th turned into Sasuke?

the 4th is probably dead, no eye to eye screenshot can convince me otherwise, don't you remember the sharingan eyes edit of inane?