Is Hanna your youngest? (in means of coming into the family? in other words, last cat brought in prior to Isabel?) If so, she is the one who has the "most to lose" in terms of the heirarchy. She will be the hardest "sell" (so to speak). But from your description things are going normal and even good, I would say. Sounds like you are doing all the right things and the only thing left to give, is time.

The growling is not necessarily aggressive. It is a warning to NOT mess with MY (Hanna's) status in the house. Isabel, will submit (at least for now.....the future is yet to be seen.....as she matures) But Isabel sounds like she will always continue to be more submissive which to me means they will be able to become friends in time.

Gee, I hope that wasn't too much like rambling?