I can't say exactly why you were able to join the revolt, without looking at the save, but claims and such usually enable other rulers to join.
As for being able to choose an ambition to become a councilor. It is a known bug that joining a revolt as an independent character, you gain access to all councilor ambitions, even though you shouldn't have these available.
Also, it is WAD that if you are a part of an ongoing revolt, you are unable to declare any new wars. So that conflict has to end before you can start another war.
As for being able to choose an ambition to become a councilor. It is a known bug that joining a revolt as an independent character, you gain access to all councilor ambitions, even though you shouldn't have these available.
Also, it is WAD that if you are a part of an ongoing revolt, you are unable to declare any new wars. So that conflict has to end before you can start another war.