I can answer a few of those questions:
Hedgehog engine just has alot to do with 2D style of gameplay and turning it into 3D and being able to support Sonics speed.
The other 5% is just like you are tails or someone else and you a small part to help Sonic or something(i don't really know but just guessing).
I think Wii version will be the most fun, but the best graphics will be on the PS3 or Xbox360, this game is suspose to have some awesome grapics, but we will have to wait a few more days for vids from E3
Sonic Unleashed
13/07/2008 / Sydney, New South Wales