I'm so sorry to hear of all the troubles you've been dealing with. You're a good owner to be treating these problems with your beloved cat. I know cats can be difficult to medicate, and you obviously have your hands full with multiple medications.
I only have a limited view into your cat's medical history based on what you've told me. The big question is how does his comprehensive bloodwork look? I have put hyperthyroid cats under anesthesia for necessary issues (dental disease, tumor removals, etc). As long as they have decent bloodwork (no severe kidney or liver dysfunction, thyroid is controlled by medication), they can still undergo anesthesia. They are at a slightly higher risk of anesthetic complication (ie. a heart arrhythmia), but this risk is absolutely minimized with a good pre-operative work up (physical exam, bloodwork, blood pressure check), safe anesthetic drugs, good anesthetic monitoring (EKG, blood pressure, respiratory monitor, temperature monitor, etc), and good fluid support while under anesthesia (IV fluids). Many veterinary clinics are able to provide this level of care. If your current vet cannot, don't be afraid to ask for a referral to a "specialist" or seek out a 2nd opinion from another local veterinarian.
Based on what you have said, if this is indeed cancer (which can be very difficult to know without a biopsy), I think the benefits of tumor removal outweigh the slight increased risk of anesthesia. ESPECIALLY if this tumor is causing pain and decreased quality of life.
If this is a tumor, there really is nothing topical that is of any use.
All of that being said, again I want to remind you that your veterinarian knows your pet better than I do. It is possible he/she knows something else about your cat's medical background that truly would preclude surgery. I would at least seek a second opinion before just giving up on this painful tumor.
My thoughts are with you...good luck!