I have to admit that this mod attracted me. Because I can drive cars in Minetest, likes driving a real car in the real world. And the mod has folding roofs, fuel systems, and car lights. But I hope you extend the maximum seat limit to 4 or above that we can make sedans/wagons or MPV/SUVs and even buses.
I hope the mod becomes better. Just keep working! : )
There's some work in the very (very) old libmount that allows mounting multiple players - which I've given a PR to tidy up. If the licences involved are still comptable, it should be possible to extend mobkit to have this feature - and thus make it easier for you to add it.