Lu, are you able to tolerate NSAIDs? The suggestions you've gotten to relieve inflammation (in the area around the sciatic nerve, which is not your SI), to help relax the lower back through physical therapy - these are key in managing pain due to sacroilietis. In my experience the dysfunction of the SI causes problems in OTHER areas - muscle spasms in your glutes (sciatica!), muscle spasms in your lumbar spine (back ache) - for me, a torn hamstring.
Developing a habit of maintaining core strength and lumbar spine flexibility, including stretching your hamstrings (gently!!!) and your iliopsoas (gently!!!!) will help keep you out of pain most of the time.
I've had friends who successfully addressed sciatica with chiropracty (using the "stretcher"). Maybe you can get referrals to a good, reputable chiro to work on this.
Ice is a terrific antiinflammatory, but to get to the bundle that's probably sparking your sciatic nerve is hard, like you say. The piriformis is deep in the glute. My experience is taking NSAIDs (I use Aleve) is most helpful in times like this. HOWEVER, as a person with IBD, please be careful with NSAIDs!