The digital speedometer has a label on it that says "SET FOR 88 MPH", so I think Old Biff could figure that out.
As for Biff coming back to 2015A, he DID. We didn't see anything change for Marty and Doc because, presumably, Hilldale is either unaffected, or because Biff is apparently dead in 2015A, the city has gotten back to normal. Marty and Doc didn't stay at the McFly house, so they didn't see that the family probably faded away as they probably wouldn't be living there.
As for Biff being curious about the flying DeLorean, he might not have known Marty was in it.
I'd just like to know why he apparently walks right past it in the game, since Marty left the car unlocked. I swear, Marty doesn't realize how valuable that car is.
"Geez, we've been back together for five minutes Doc, and you're already talking about the end of the universe."
"Stupid, stupid rat creatures!"