I never advised anyone to keep a part of their rectum, I explained my experience and I suggested this maybe a route the OP could explore.
The first surgeon I was referred to; he painted a very grim picture of life with a J pouch, he even suggested I keep my stoma because I had adapted so well; are you saying this surgeon knows best ?
The surgeon who removed my bowel and created my stoma came to the conclusion that I was infact suffering from Crohn's and not UC; it was his diagnosis that prevented me from undergoing j pouch surgery for so long, did this surgeon know best ?
If FAP is as grave as you say; why did the Surgeon leave 5% of the OP's large intestine, when there's such an extreme risk of cancer; also, isn't the rectum retained during J pouch formation ?