I have done a little test. One great thing is that you can chose for heir a character not come of age. I have killed the king in battle, and the child was next himself designated as king, despite not being still present on the strategic map.
Thus, there was no faction leader during this time:
But the character has come of age while being king without apparent problem:
I have also tried to make heir spouses and daughters . It should be disabled i think, since, even if it has no real effect, the message appears and it removes the last attribution of the heir title. Finally by chosing a child i have met a bug where i had two heirs:
I don't know how i have done, maybe it's because the child has 0 for age as you can see? By designating a spouse i have been able the remove the title for the other character, but no way to remove it for the child.
I have also managed to designate the king as heir, while still having the child as heir, and so no more apparent king:
And next, when i have removed the title of heir to the king, there was no more faction symbol on his card, even if he was still king:
PS: The bug report topic is closed.