I dealt with a similar situation, with a larger fistula that would pass stool (and often had particles get stuck in it). Sadly, my first advancement flap surgery failed (I knew fairly early on but we held out hope for secondary healing).
I have since had a temporary (hopefully) ileostomy and opted for a gracilis muscle flap, which seems to have mostly done the trick (nearly fully closed).
On the prep, enemas were always hard as I would leak immediately and had a harder time holding it in. I'd perhaps treat the area around with hemorrhoid cream or similar before you start and now that once your colon is mostly cleared (which happens quickly) you will just have to deal with liquid passing.
Best of luck...and rest up up and stay positive!
Diagnosed with Crohn's: 2002
Past Meds: Pentasa, Remicade, Cimzia, Asacol, Cipro, Flagyl, Lialda, Humira
Current Meds: Stelara, Azathioprine (100mg/day)
Surgeries: Fistulotomies & Seton Placements/Adjustments (9/2011, 1/2013 & 8/2013); Advancement Flap (12/2013); Temporary Loop Ileostomy (7/2016); Fistula Repair with Gracilis Muscle (9/2016); Fistula Repair (12/2016)