I am sorry to hear about the passing of your cat.
20 years old is a very respectable age for any cats! That is awesome, you have taken good care of her to go so far.
I will not be able to tell you exactly of what your cat died of, but I can give you possibilities from the information you gave me.
Bloody nasal discharge is what is striking me the most with he face swelling.
She might have had an abscess from the tooth, it got into the bone and eventually in the sinus. Let me put a link here about dental disease:
Or she may have had a tumor in her mouth that spread into the sinus cavity. We do see Squamous cell carcinoma in cat's mouth, it is a very invasive cancer and destroy everything in its path:
The fact she did not eat much could be form a painful mouth. Cats are very good to hide their signs of pain, it is a survival instinct. Pain is graded form 1 to 10 with 10 being the worse. Some mild to moderate signs may not show much.
Or she may have had some other disease process decreasing her appetite. Having the hyperthyroid condition will make them loose weight that they are eating or not. So it probably contributed to her weight loss .
Let me know if you have more questions, or need additional information.
I am sorry again for your loss, remind yourself that you had her for 20 years and you did your best to take good care of her.
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Thank you for your question, and take care.