I'll take a stab at your questions:
1) Not over the seat back; but yes, a skinny person could possibly squeeze through the window partition.
2) Most likely as that seems to be standard procedure. Though, flexible people "slip their cuffs" all the time...simple google search.
3) Yes, most likely running. Ever seen a cop shut off their vehicle to do a traffic stop? No
4) I believe one of the statements said that Officer Armstrong was injured when McComas hit the gas to steal the car.
5) It would seem so.