Maheeru or Tui, please help with multiple issues in senior cat

Dear Maheeru and/or Tui,

I would like to ask your help with my 16 year old cat who has many issues with which the regular vet cannot help much.

She is a female neutered 16 year old Calico cat. She is small (approx 2,5 kgs). She is a very friendly cuddly cat, likes to cuddle, lay on your lap, purrs a lot. She used to be much more scared of people, but in recent years she is much more sociable.
She used to get bullied by her brother a lot (he passed away a few months ago); she gets bullied by our other cats (I have 4 cats in total) sometimes but not too bad.

She hardly had any vaccinations in her life, only when she was very young. She used to get spot-on flea medication in the past, but didn't need it for many years until recently. (I will expand on that later)

When she was about 10 years old, she had recurring idiopathic cystitis. This cleared after about 2 years, don't remember exactly how long. She did not get antibiotics then, because it was not a bacterial infection. The cystitis was probably caused by stress.

Her recent health problems started in the beginning of 2018. She was losing weight despite eating normally and she was being very vocal and hyperactive. She also had a bad odour coming from her mouth and had nasty dark gunk stuck on the sides of her mouth on the lower lip.

I took her to the vet and they looked at her teeth and she needed dental cleaning and extraction of at least one mollar. They first did bloodwork and this revealed she had hyperthyroid (hence the weightloss and hyperactivity). Also her kidney and liver values were not good and she had a heart murmur.
They put her on thyroid medication and advised an ultrasound to see what might be causing the heart murmur. The ultrasound showed hypertrophic cardiomyopathy but no chronic heart failure. She got medication (ACE inhibitor) for her heart, which would also help her kidneyfunction. The vet thought her kidney-, liver- and heartproblems were caused by the hyperthyroidism.
They also advised to put her on a kidney-diet.
The dental work was put on hold because of her age and condition (anesthesia would be risky).

I put her on the kidney diet (canned food), she did eat this for a few weeks, but her appetite got worse and worse and she refused to eat the special diet. So I had to put her back on normal canned food (good quality) and also gave her raw fresh meat as much as I could. Her appetite remained bad, I had to switch brands of food regularly because she just wouldn't eat it anymore.

Her dental problems remained and she usually had gunk stuck to her lowerlip and also very bad breath and lots of drooling.
She also started to have recurring coughing and a lot of yellow snot coming out of her nose and sneezing. Her nose would be very congested. Also excretions from her eyes, which gave her dark corners of the eyes.
She received, over the year, several courses of antibiotics for this, but every time after about 2 months, the coughing and sneezing/snot would return. Her nose became also very dirty looking, with dark crusting on it.

Recent bloodwork showed that her hyperthyrooidism is under control with the medication and her kidney values improved a bit.

About 2 months ago she developed very bad itching and many crusts on her back, about halfway on her back towards the tail and mostly on the back, not somuch on her sides and nowhere else on the body. First I thought it was an allergic reaction to something that I had been giving her for 2 weeks which had pork in it, which she never had before. So I thought it was an allergic reaction and I gave her Ars. Alb. 30C, wet dosing and succussing every time I gave a dose, for about 2 weeks but it didn't help. Then I tried Sulphur 30C for a week, also no help. The itching was really bad for her. I combed her back and saw black spots, which resembled flea droppings, so I got one spot-on thingy for fleas from the vet and applied it to her. I hated to give this to her, because it is very bad stuff, but I didn't know what else to do. I also gave her 1 dose of histaminum 30k.
This did do the trick, over the next few days the itching became less and after a week I could comb her back to remove all the crusts that were still there and this has mostly cleared up now.
It is still a bit itchy, but no more crusts.

Her mouth, however, is very bad. She drools very much, and it is slimy and makes long strings dripping from her mouth. It smells really bad. The dark gunk is also still there and she has developped raw skin on her upper lip (left side) and on the sides of her mouth on the lowerlip, where the gunk always is. I remove the gunk and drool as much as possible, but the area is very sensitive and she doesn't want me touching it.
When she lays sleeping, she is roaled up into a ball and then she drools all over her tail and her back paws. They are therefore also extremely dirty and stinky and she hates me cleaning it. Yesterday I gave her Calendula 30C, 1 wet dose, because of the raw skin/wound on her upperlip and lowerlips.

Her appetite has improved since she got the Ars. Alb. and Sulphur. She drinks a lot (has been doing that for about a year). Some days she is still very vocal, other days she is quiet. She mostly sleeps during the day. She does come to cuddle a few times a day.
She still likes to play with a toy on a string occasionally.

I think she also has something like arthritis, she walks a bit stiff with her hind limbs and she is not able to jump to higher places anymore. She can jump to a place that is at the same hight as she is. She doesn't really groom herself anymore so she has a dirty appearance.

She had something weird regarding her back limbs about 10 weeks ago. She would walk with a bend in her back, with her hind limbs going sideways, it's hard to explain. It was like the back part of her body was bent to the right and she was not able to walk straight. This lasted 2 days and then it was gone, with no intervention.

I hope I have described my cat and her problems well enough and that you can help her. I feel her immunesystem is very low. I have planned an appointment with the vet to see whether dental cleaning under anesthesia is an option but I am afraid to have her put under anesthesia.

Thank you in advance for your help/advice.

kindest regards,
Louise de Jong