The answer is in the question: "...he sees many many people, old friends ..."
Adam and Eve are the names of two of his friends.
Yeah that makes sense. I was thinking [with my cryptic response] that Adam + Eve would be the only two people without navels.
As for your theory one, I couldn't think of any note on the piano that you'd have to play with a specific finger, except perhaps you might argue the lowest / highest note you might want to play with the pinky rather than having fingers off the edge of the keyboard, hence I figured it would have to be the lowest note to have any reasonable answer to the bonus [although I'm still not convinced by it]
If there's some theory that gives a better answer to the bonus I'm intrigued.
Similary for the theory, if being on the right is significant etc. If it's not a C then perhaps A [I assumed a trick element like some of the other puzzles, hence picking the bosen with its lower bass notes] Perhaps the pedal is down too, so some of these higher notes are symp res from other strings?
But I figure googling for the answer is pointless - especially since most of the other puzzles will be easily solved that way, it'd be too much like looking in the back of a crossword book.