I'm so sorry things aren't going well right now. I don't have any words of wisdom as far as what to do, but I can share our similar experience. When we brough our Chloe (cat) home from the shelter, she looked fine. The next day, she started showing signs of being sick. She got progressively sicker over the weekend, and we really doubted whether we had done the right thing. (Her throat was so sore and swollen that she was foaming at the mouth.) After treatment from the vet, she began, slowly but surely, to show improvement. She is now a wonderful pet. All this to say, with the proper treatment, Muffin should be fine. Just be patient. (Easy to say, hard to do, I know.)

Hopefully, things will get better very soon. (((((Hugs)))))