I don't really know, tbh :-/
But it sounds like you have Crohn's throughout the colon? If so, leaving half a diseased colon behind doesn't sound like the best idea ever. Even if you don't have symptoms now, you could become symptomatic in the future. But, more to the point, reconnecting to diseased tissue massively increases the risk of an anastomosis leak - to 50%, according to the colorectal surgeon I've spoken to.
I also know that in Ulcerative Colitis patients, the entire colon is virtually always removed, even if the patient doesn't have pancolitis (colitis throughout the whole colon). I know; different disease - but it still has a lot in common with Crohn's colitis.
The cancer and the PSC I can't really comment on; don't know enough.
I can't, in all honesty, tell you what your BMs will be like after a colectomy :-/ I'm facing one myself, but I haven't had it yet, and it would be different to yours anyway, in that I would also have the rectum removed and a stoma put in.
This study suggests that 72% of patients are still going strong 10 years after an ileorectal anastomosis - not too shabby odds, in my opinion, but still not very consoling if you happen to be one of the 28% who need a reoperation.www.ncbi.nlm.nih.gov/pubmed/21979177
Sorry, I don't think this was a fantastically helpful post, but I tried :-/