The governor titles in BC are transferable ancillaries - so you can swap them to a younger FM if the holder is reaching a grand old age - just pop the new guy in the same city or stack and click, drag and drop them them across onto his unit card.

I think (but haven't tested it), that even if the ancillary dies with the holder, a new guy can get it by spending a sufficient amount of time governing that city.

I don't think a vassal can attack their liege if the vassal is AI controlled - but in hotseat play there is nothing preventing it..