Theory or not, incest occurances cannot be denied when it comes to lion life, you know. Like if a adolescent male is expelled from his native pride, but later on returns to power, complete with mating rights over all the available lionesses, isn't it just natural for disney to even allow for speculating about the matter in the first place? True, disney hasn't officially stated that Scar was Nala's father, or that Nala was Scar's forced mate. Albeit for a short while) Yet I haven't seen disney denying this either as far as I know.
So both Mufasa and Scar could have quarried over who held the rights per individual lioness in heat. as they were the only males within the pride, ánd being brothers aswell..Hence Mufasa's tolerance for a second (potential rival in mating season!) male within his pride... And I believe a rogue lion couldn't be Nala's father, as rogue males don't have much chance to enter a male-guarded pride unseen, mate, and dissappear again. Besides, all animals simply follow their basic instincts, and one of them is to live and conceive babies in order for their species to survive..It's an undisputed fact, that it sometimes goes the wrong way though.. It is as simple as that.