My family has always used Cosequin for Cats for our older cats that develop arthritis. It's fairly inexpensive and has always had great results for us.
My vet normally recommends antibiotics prior to a dental cleaning. He's said that he generally requires it for mouths that are going to need a lot of work. As far as the medicine, we've always been given liquid clindamycin for the kitties.
I wouldn't think that the injury could be from the declaw, considering you said it has been so many years. She most probably just hurt her paw slightly jumping. I hope she gets feeling better!
It sounds to me like maybe she has allergies or some other condition on her tail that is causing pain or discomfort. I would check for any signs of her chewing (like with allergies) and if nothing, I would get her in to see the vet in case there is truly an issue with pain.
I would talk with the vet and voice your concerns. It's definitely a valid worry if your kitty has a weak immune system. In this situation I would want to rely on the vet's opinion of whether or not he is healthy enough to handle the shots regularly. If the vet says it's okay, then I would agree.
I had a cat that had a tumor in her mouth - the vet removed it and although it came back a couple of times requiring more minor surgeries, she lived another three years. I'm sure everything will work out perfect!
I wouldn't worry about it, unless you mentioned to them that you would want to do a biopsy if they found cancer during the necropsy. Did the vet acknowledge that he found tumors after he performed the necropsy?
I have been giving Metacam to me and my parents' pets for a few years now, including the cats, and have never had any kind of adverse reaction. Even the box now says that it's safe for cats.
Most cats don't need e-collars after being neutered or spayed. As long as she is okay with leaving the incision alone, I would definitely say she doesn't need it on while she's around you - when she's alone would be up to you.
Although I understand you being worried because your kitty is so lethargic now that he's back at home, urinating where he lays may lays may be from having the catheter in at the vet's - he may be having trouble holding his urine.