In reality the concept of "Noble Republic" seemed always imprecise to me... in EU3, if I remember correctly, you could do Royal Marriages with them but never get a PU!
Now it seems possible to obtain a sort of Personal Union that being an elective Republic has not a lot of sense...
The reality is that IMHO the Noble Republic models two different type of governments:
- Aristocratic Republic as the Italians city states: Florence, Pisa and so on... No royal Marriages, the ruler is a Prince and it is elected any 8 years. They could evolve in a more authoritarian form similar to Republican Dictatorship (Signoria in Italian) and then in a proper Monarchy.
- Elective Monarchy as Poland and after the Polish - Lithuanian Commonwealth, Hungary, Bohemia and so on... The King rules until death, there is the possibility to have Royal Marriages and to form PU. On Monarch death there are elections, but a new choice is present if in PU "continue the personal union", the IA could be instructed to chose it if the relations are good... it pretty obvious that the player would escape from PU really easy in this way! But the restoration of union CB would make sense again...
Regarding to the problem that minors in Personal Unions could escape from the union becoming Bureaucratic Despotism or other forms of Republics this should not be permitted to happen! If a revolution tries to overthrown the ruler, a sort of Civil War should start in which the Senior of the union will be called to defend one of his thrones... if the war is won the union continues as the revolution is ineffective if the war is lost and so the Revolution has won and so no restoration of union is needed...
Let's try to imagine what would have happened if France tried to become a Republic being in PU with England! Do you think that England would have done nothing?