I'm mainly surprised by the "we" used by Satoshi: "We propose a solution".
That's probably a lot more easily explained than some of the other mysteries around Satoshi.
In scientific papers, even when they're written by a single person, it is not uncommon to use "we".
I don't know exactly why, but I've seen it quite a few times.
May have something to do with the fact that in many cases there are at least a few indirect contributors to most scientific work.
In short, it's probably just an indicator of a scientific / an academic background of Satoshi.
Because of his sudden disappearence, I always assumed Satoshi must have been one person.
One of the most likely explanations for that may be that you will have to differentiate between the Satoshi who "invented" Bitcoin and the persona Satoshi who actually wrote the code and was active on the forum.
There may have been a group of people who collaborated to design Bitcoin, but only one of them was a programmer with some decent skill.
After the initial design had been laid out, the others may have stepped back, leaving the programmer alone with the dirty work.
In the end, the single "public" persona Satoshi disappeared, whereas the people behind never actually appeared in the first place.
AFAIR, Gavin received some emails from Satoshi for quite some time after his last post.
So, he didn't disappear nearly as abruptly as it seems.