Are jewish rabbis NOT disqualified from inheritance

Right click on the holding and convert to vassal. Your character isn't technically allowed to 'run' a city or a church. (S)He may only run Baronies as they are literally in the royalty hierarchy.

Why is this? Cities want to elect their own Mayor so the townfolk would prefer a mayor to a King. The church thing is a bit more complicated but until you become a King, you have free reign over who succeeds as a given church's bishop but once a character controls the holding the usual problems for removing a title crop up.

A second note on the Bishop problem, if future readers are curious, is that once your main character becomes a King (s)he has the issue of crown laws and the subsequent issue of free or Papal investiture. You'll have to look more into the consequences of either choice but the relevant issue is that under Papal investiture your King has no influence over Bishop appointments.

Yes, other non-descendents can inherit.

Think of it this way: they are a descendent of the previous holder of the title (the current ruler's parent), so they are already in line for the title. One brother inheriting doesn't remove the other's claim to the title, so the second brother is the first brother's heir, at least until the first brother has children. Then the second brother is still in line for the title, after the first brother's children.