Yes we have oil heating here too...........flaming expensive it is too grrrrrrr.
are your cats chipped, cos this would be a proof of ownership if Bronte was being fed somewhere else. You could put a paper collar on her, saying please do not feed she is has health problem.
I think the introductions should be completed fully before doing anything else about Bronte, if you have been keeping her in, however I suspect you should have allowed her to have her normal life and restricted the newcomers.
She will be angry that the newbies have come into her territory and if she isnt having her normal life will think that she has done something wrong, especially if she was nervous before.
I have to say I got two birmans about 6 months after the big cats and the 4 just do not like each other at all. But they live in the same house moreorless peacefully as long as someone does not deliberately try to steal someone elses normal place. They were introduced very carefully but even so it was not totally susccessful.
Wizzs instructions to me and for me as for you, it was too late! but the newbies need to be kept in one room until they are happy and must not meet the other cats. Then when they are happy in that room they should be moved to another and the residents allowed to visit the other room minus newbies. Once they are all happy and the residents will go in and out of that room, then you repaet with another room, until newbies have seen every room in house and residents have experienced newbies scent in every room.
Then comes time for the cats to meet under supervision. As long as they do not fight and there are no more than minor hisses fine but if they start fighting then you need to start all over again with intros from the beginning.
Mine met by accident on the first day snd it was immediately growls and hisses and running and hiding by my resident cats and the birmans who were much older were fully confident in every room immediately and one of them used to dig up the carpet behind the door at night, and I would not be able to get back in!!!!!!!!!!!! I resolved that but my prob was that residents were nervous to start with and newbies confident, notr the best partnership and its never worked purrrrrrfectly but it works within reason.