I too have a deep fistula that exits in a similar area. I had extensive surgery 6 months ago and to date I am healing resistant. Meaning I have an oversized fistulotomy trough stretching my entire perianal area that won't close. I have a seton in place - a band that goes through the former area of abscess and back to the surface - that keeps me from healing prematurely down deep and thus reactivating the abscess. As a result, I have no deep down abscess agony, but rather a lingering nasty surface wound to manage. Having developed an allergy to Remicade we are trying Humira as an option for healing the wound and I have had some modest improvement with fistular drainage going way down and the wound itself decreasing very slowly in size. So, I very much understand your situation.
I am unsure of some of your details. Do you have an active fistula with an external drainage site? From my experience, it sounds as if you have below surface fistula activity and that a tract is burrowing its way to freedom. If an MRI shows tracts how can a surgeon refute that? An external exam only identifies whether or not your fistulous tract has escaped the body? Have you tried Remicade? Good luck.