Hey it seems you had a complex work around the physics car behaviour. Sometime ago i tried to make a soccer type ball physics behaviour from scratch, but unfortanly i didnt make it. Its a very complex task and my physics knowledge dont help much.
About your collision with environment problem, if i understood well the problem shell be the physics that controls the collision with environment, isnt it?
And not the collision system / hull i believe, because if you use c_move you will not have problems on that matter i think.