Well , In this particular kingdom , there was only two possible candidates , since only two male lions lived. the thing is , One was appointed to become king , with nala as queen. Simba was the son of the king , thus , as the only male with royal blood , he had to become the king. As you pointed out , that's how it worked
then you mention qualification , but the kingdom was snapped out of mufasa's hands (and simba's) by Scar , who had time to try his hand at ruling. See how that worked out ? Everything died and even the royal family starved. So i guess simba was at least as qualified as scar when it came to ruling
Then the ghosts. Well , using the stars as fallen kings and allowing mufasa to become a ghost was genius. First off , it was a way to set up a scene where we saw simba and mufasa lecturing , then playing. We , as spectators were shown the soft side of the royal family and we became attached to them. That made the scene that comes not long after way more dramatic . Afterwards , something had to make simba come back to the pridelands , something strong and emotional which also affected the spectator. that's why mufasa was bring back as a ghost. (i do admit i am quite sensible to the part where mufasa's ghost explain to simba that he is the king , then vanishes)
In short , i believe that they just
used the divine right , to set up some scenes and because in an animal kingdom , that would seem kind of natural. the first lion cub becomes the next king , it's smooth and easy to understand. I do not think they "defend" this way of ruling at all.
edit :
this essay for exemple take it too far , it wouldn't have worked to have anything but a royal family in the plains of africa. It is just entertainement after all