i am sooo glad to hear she has been to the vets. Like I said before, I don't know about rehoming dcats but i am disgusted and deeply saddened by this rehoming centre's treatment of a cat, and what its done to you too MrsMoggy.
You were worried for good reason and should have known you were taking on a dcat with flu, no wonder she's hissing and sneezing and she should never have let her go in that state (she obviously didn't want the other dcats to get it and bring huge bills - i cynically assume).
Oh.. .I 've just seen your post! I am so glad you are fuming, you've every right to be after her treating you both this way.
She actively allowed this dcat to suffer all this time, and i don't think you could have had any way of knowing whether this dcat's hissing was down to being scared or illness, which is why it was so important to have it done at the start.
You have trusted your instincts and taken her upstairs, and kept her near you. I imagine that ball is matted fur, and she sounds like a kitten farming dcat possibly who's lucky to have been dumped into someone's hands instead of in a ditch. so sad for you both.
She's very lucky to have had the time with you that she has, in peace and quiet with good sleep, environment and food.
rescue centres have a responsibility to educate those coming to them for fosters, as the animals there need that extra security, but she should also be responsible for taking it back, because thats normally a thing too, that they retain ultimate responsibility for it in case of future rehoming, change in family circs, or death. so alright, you could have done all the research and found all this out for yourself, but you went to a homing centre, where you felt led by their laughably 'superior' knowledge! so assumed it to be ok.
you have put your heart and soul into caring for this dcat and worried yourself to a frazzle in the process, you couldn't have tried harder and are lovely for wanting to do the absolute best by it.
God forbid that any of our animals would ever need to end up in the hands of unscrupulous rescuers like this!