Is it permissible for the ownership of the car to revert to me through the waiver of the heirs, after the death of my friend in whose name I temporarily registered it, with the heirs' knowledge of this agreement, or is it considered their legitimate right, and they are not permitted to waive it?

If the fictitious registration of car ownership did not transfer ownership from the original owner, then the car remains his property. The heirs of the other party have no right to claim it after his death merely because it was fictitiously registered in official papers in the name of their deceased relative, especially if they know that the car belongs to the original owner.