I hope you have been informed that vomiting and diarrhea are extremely common with gastric bypass, so you will probably have an increase in your current problems rather than a decrease--with or without removal of a potentially faulty gall bladder.
I was reading something a few weeks ago about chemotherapy and children, and the fact that they are finding that chemo causes a lot of problems in kids later in life--especially bowel problems. I wouldn't be surprised that your cancer and radiation treatments both have led to your developing problems with your intestines now.
Please know that this is my own, personal opinion, but if I were you, I would be reluctant to reroute my intestines and stomach when I'm already having problems with them. And especially since you have had cancer, and your body almost certainly isn't healthy/functioning correctly now due to treatments you had in the past. Having lived for 5 years with a bad gall bladder, I am terrified of being that sick again. I wouldn't wish that on anyone; and yet, from what I have read, constant stomach/gut problems is pretty well par for the course with gastric bypass. Of course, the fact that 1 in 50 people dies within a month of having it done is pretty freaking scary as well. I would think that, given that your body doesn't work correctly now, your risk would be even higher than average. Contrary to what diet doctors preach about fat killing, most fat people--even most morbidly obese fat people--don't have that great a chance of dying at any given time. I know people who have gotten it done and like it (even though it never makes them fashionably slim), and I've read some real horror stories about how it ruined people's lives and even killed them.
I'm sure you've looked at the lap-band versus gastric bypass already, but if you didn't know about the bowel complications that GB can cause, you may want to rethink the lap band. I have heard it doesn't cause anywhere near as much diarrhea and vomiting and, of course, it's reversible and has a much, much higher survival rate--on par with any other routine surgery, as far as I am aware.