I am struck that a basic question has not been asked. What is the cause of the diarrhea. The assumption was made that this is XRT induced. Perhaps something else is involved. Is it secretory (does it remain despite fasting??)? You've already shown, eloquently, that it is "small bowel" and not of colonic origin. Is there malabsorption?? Are we missing an endocrine tumor?
Before further surgical endeavors, I think that this type of evaluation must be completed. Use XRT enteritis as a Dx of last resort. You need to do 72 hr fecal (chyme?) fats, Serum and fecal electrolytes/osmolality, response to in hospital fasting, hormones (serotonin, 5HIAA, Gastrin, VIP, etc.), small bowel bx, search for seripiteous laxative use, etc.