This is a very painful issue
Been there done that twice, The vets you went to, don't sound to educated in fistula's.
Check out this site perianal-fistulas
And join their email list.
You've got a couple of options, different medications..One I would consider is removing the anal glands. BS that they are telling you surgery can't be done because it's not 'bad' enough?
That site should list vets in areas that are "up" on fistula's.
Another thing you can try is Desitin, as in baby diaper rash creme..Shave up the area, keep it clean, use a Q tip, insert Desitin IN and around the fistula(s) twice a day..
Desitin isn't a cure, but it may relieve some pain, and can possibly close up the fistula(s) depending on how bad/far gone they are. Canned pure pumpkin added to food can aid, (1 heaping T each meal)
Your best source of info is to go the link I posted above and I'm moving this thread to the health section..