Sadly, I think that the "pocession" thing wont be possible for very long: the french 1635 cards give him a "imunity: controled". Wich I think is a god thing, it is silly to loose nearly half your liste just beecause of a spell (and fyi: I play demon too!
)!
For her healing abilities, I don't think that will work very often: she has to be activated to be able to heal the GDA but with the damage Bran does and the two fights the GDA has to doo in his own turn, she won't have time to eaven do a vae and her healing order: the GDA will already be dead.
It can work with Samael, but you'll probably lose him in the fight too...