the core module of fgear has some approximations but overall it is realistic, without external forces it is difficult to make games like trackmania or rollcage. you need to get your hands dirty with physics and use external forces to get the behavior you need.
1 - normally that shouldn’t happen unless you force the vehicles to penetrate somehow, can not judge without visuals but this sound like unrelated with the vehicle system.
2 - giving unrealistic friction values can result such behavior, you need to compensate with another unrealistic modification like lowering center of mass or just like your way. i do not understand what you mean by “throws the long/lat slip out of alignment”.
3 - you can modify the raycasting filter to skip those objects or you can use multiple raycasts to avoid instant height changes or you need to modify the suspension code to compensate for instant height changes.
about other questions:
1 - changing wheel radius and friction is possible.
2 - this is not directly related to the vehicle system. i have never tried such change in unreal but i guess you need to reapply some settings after changing the collider so that the inertial properties of the rigidbody can persist.
3 - i think wall riding is possible for realistic conditions but the tire model is not sticky at all, you may not get a driving fee like “on rails”. for unrealistic wall riding, you will need external forces.