That said, we are very deep into the game and I think it would be a bit wrong to change all of the vassalge rules at this point.
No that's not needed. But we somehow need to ensure that a Ruler can properly punish his elector if he says that the elector is banned from the Diet, for example.
If the Kaiser bans someone Econ will simply delete all other posts, however if the King or a Duke does this the Elector can simply say: screw you, and post anyway.

In real, this person would be as low as a peasant. However here he can still come into the Diet and even vote.