Group or individual?
Both!
I believe the lions share of satohsi's work was done by a single genius.
Certainly he was helped in places, but I think that was sparing rather than a proper group effort.
Nationality?
Almost certainly a british mathematician.
There's clues he has lived in germany and japan. His gmx address (common in germany) is linked to his public GPG key.
Background?
Satoshi is the top class of top class mathematicians. He's also an incredible programmer. He's certainly been around the crypto comunity for many years. He has good competency in accountancy and economics.
Education?
You'll have to trust me on this one. The British school system is such that almost all of the top mathematical talent are sent to Cambridge. In fact, to a specific college, Trinity. Satoshi was almost certainly of this calibre. I know this sounds speculative, but it would be like predicting Messi played in La Liga.
I wonder if he did a PhD
Satoshi has also self educated in many fields and been in and around the mailing lists
Age?
Unknown but almost certainly above 30. The 37 age he gave may be close, so he'd be around about 40 now.
Profession?
This is the hardest of all. Did he become an academic? Did he work in a big accountancy firm. Or an investment bank. Did he become a programmer.
Almost no one knows but him ... and maybe a very small circle ... I'd love to find out, and look forward to his return!
Great post!