That part you keep removing and adding back isn't attached correctly. You can see it moving up and down as you accelerate or brake. The black axels aren't really connectors. I think it needs a wooden connector to be firmly attached and your klang problems will go away.
Your issue with the car turning upside down come from the component you use to link the engine together, the game try to put torque on it because of the material (it is a bug) that make it rotate the engine line which rotate the car... Try another material at the start and end of the line it should prevent it from happening I think also connect the block correctly For the folding wing, you can use a piston or a servo to push it with a pivot point depending on the design you can fold them or just rotate them up or down or left right if you add straight piece on the wing
I dont know if you fixed it or not but the car at the beginning, Mk III, is only 3 wheel drive. The rear left wheel axle is too short to reach the wheel.
by definiton, ignoring friction losses, if you transmission has a 1:1 gear ratio, theres no reason to put your electric engines on the driveshaft, its a waste because it won't provide any benefit at high gear. if you're going to bypass the transmission, its better to put it on a high gear so its initial torque can still be used at low speeds but the higher back-emf limted range won't come in until youre up to speed