very nice video, congratulations.
1-it already works with ue5 and physx, the answer depends on how unreal engine will support both engines, if you will be able to switch from settings then we can support both but if one of them requires separate source compilation then we might stick to the default one. also if epic decides to depreciate one of the engines then we will also do the same.
2-by default standard input component uses combined axis input. in order to throttle and brake at the same time uncheck “Use Combined Axis” setting.
3-gear change takes both wheel speed and body velocity into account and there is a small threshold between 1st and R gears. this default behavior is safe and works fine but if your game requires more aggressive gear changes then you might need to modify the source code.