Few Questions

I am not sure I understand the feature of having your companion become an aid.  Does he no longer fight for you?  How is becoming an aid beneficial?

Can you leave a faction after you have become a vassal?

Making him an aid allows for his support skills to be utilized, while he's kept out of battle.  So you wouldn't suddenly lose the ability to track because your ai partner is an idiot and got cluster-****ed by a band of sea raiders.  It seemed to be broken the last time I tried it, although to be fair I haven't tried recruiting an aid for the current version.

You can leave a faction, but it has to happen when your leige refuses to give you a castle or fief after a conquest.  Otherwise, there's no way.