According to the last (and only) royal Constitution (1928) only the King's male descendants can succeed. His half-brother Xhelal had no succession rights and so Xhelal's sons don't have them either. In case there were no direct heirs of the King, there were various provisions in the Constitutions but they all required a voting in the Parliament. So Prince Leka is currently the only person elegible to succeed accoring to the old Constitution.
As for the titles for the King's siblings, again according to the Constitution, they could not be passed to their children.
I wondered why, in P.Theroff's site, Pr. Xelal's line descendants were preceeded by Pr./Pss with a ? Question Mark.
I guess that's the explanation. They are not Zog's descendants, so they are not "proper" princes and princesses.