1) Woods can be considered open woods. If you want more difficult but still traversable ones - add rough under them. For really nasty ones then you could possibly use mud as a base terain - thus giving a chance of sticking.
In fact - I cannot think of any wargame rules (paper based or tabletop) where woods are totally impassable to vehicles.
2) I am not really sure of your point here, I'm afraid.
3) The SP system uses a simple size variable - and a T-5X is usually size 4, wheras a ceturion is a 5. (If the original code had been based on tabletop rules (I think it was based on panzerbliz, a hex based boxed game ?) then it might have been similar to the Challenger/Firefly rues where vehicles get an aspect such as A/C/C - cannot recall which one id the side on, ahead and turret size aspect (havent browsed those rules in 5+ years
. But then again - the WRG rules had no size modifiers.
Cheers
Andy