Hi Pascal,
The vehicles in UE4 are driven using the PhysX vehicle setup (https://developer.nvidia.com/sites/default/files/akamai/physx/Docs/Vehicles.html), and I’d hazard a guess that the implementation somewhere along the line will assume that Z is down - this is manifesting itself in 1) where it appears to be only taking Gravity in Z into account, and not forces that would otherwise be applying it to a surface.
In the case of 2), AddForce() and AddForceAtPosition() should be able to affect the vehicle via a primitive component (the mesh component?). If you’re unable to get this to work, I would consider that this may be a bug.
Unfortunately this probably isn’t the answer you were hoping for, but I would consider that the unique requirement you have for your vehicles may mean you should instead be considering building your own setup for driving them rather than relying on the default PhysX implementation.
On the other hand, the vehicle systems are also still fairly ‘in development’ and have changed significantly between builds, so it’s possibly that some functionality you want hasn’t been exposed yet (and other functionality may be subject to change).