Hi and welcome. Well...strange as it sounds...I loved my seton. But, I will caveat that with my fistula was perianal with very limited muscle involvement-to qutoe the nurse "if you have to have a fistula, this is the best place".
Can't help much on interpreting the MRI. Based on my experience, I would ask the doc "does this mean a lot of muscle involved?" The less muscle...the less issues it seems.
My seton was considered surgical, but you would never know it. I had the seton in and it was very tender in the area for about 5 days. Then somewhat touchy to sit on for about two weeks. After that, I barely noticed it.
I will say that if you don't have a sitz bath, get one! Warm soaks were very comforting and they gave me a cleanliness I really craved. I also took full advantage of my hand held shower head (we started calling it the naked bidet)
Hope your seton is as good an experience as mine was.