Sacroiliitis: this can be active IBD related or not, or both. This means that you can flare because of active IBD or it can have a completely independent course. If this is enteropathic arthritis (a type of spondylitis) then it is both associated with IBD, but still may have an independent course. If it is ankylosing spondylitis, it is independent of other autoimmune diseases you may have. Either way, the treatment is the same. NSAIDs are a first line of treatment, but can be problematic for those with IBD. You can also treat with sulfasalazine, but not if you are intolerant of sulfa (drugs like mesalamine don't work). Plus, it is mostly for the peripheral arthritis, not the spinal forms.
They are opting for biologics sooner now, as the best option. Enbrel is OK for ankylosing spondylitis or psoriatic arthritis, but not so much for enteropathic. I was on Humira for a few years. It worked OK for me, but I had to inject weekly instead of every other week (more common if you have enteropathic). Plus, it would sting like crazy. I've been on Simponi for about 5 years now and I like it a lot. Once a month injections, and no pain with them! Less frequent injections make it much more convenient if you like to travel, since they must be refrigerated.
Since I started Humira and later Simponi, I very seldom get pouchitis flares, and when I do, they are pretty mild. I still have back and joint pain, but it is much more manageable now. Humira is approved for both Crohn's and CD. Simponi is approved for UC, soon will be for CD too. So either of these would be effective for both the inflammatory arthritis and IBD.
With biologics you need to be tested for TB and you need a battery of blood tests every 3 months to monitor for bone marrow and liver dysfunction. Nearly 10 years after starting them, my labs have only gotten better.
As for the Crohn's, do not underestimate how damaging NSAIDs can be to the gut. I actually had cobblestoning on my rectal cuff that looked like Crohn's, but cleared up after stopping NSAIDs. You cannot accurately evaluate for Crohn's when there is NSAID damage present. That said, no matter how many times you get a negative Crohn's report, it does not mean some day they will eventually make a diagnosis. Still, it matters little, since it would not change your treatment.
For more information about the sacroiliitis and treatment, please see the various pages in the link in my previous post.
Be well!
Jan