Here is my thinking. You can take or leave it, since I am not in your shoes, and only you can really decide whether an observation fits your situation or not.
I think you are focusing on how much time has passed since colectomy, or take-down, or whatever. Perhaps too much focus in that direction. The reality is, or at least was true in my case, that each time there is a complication that upsets or interferes with progress or adaptation, it is like starting over. Or it is almost like that. It is frustrating, but you cannot make the clock go backward, only forward.
So, I would suggest focusing on progress made since your most recent setback, diagnosis, and new treatment, which has been three days. Not much time to expect healing and to be eating a more normal diet. You would be really surprised at how long you can go on a very limited diet. You say you cannot tolerate Boost, but I cannot remember what you said about Ensure. There is nothing magical about eating solid food, and you should be OK with a liquid diet until things settle down with your butt. With your history, I would not encourage you to be adventurous until things were healed better downstairs.
The IBS is a different issue, and colectomy does not really solve that, unless your dietary intolerances were purely due to UC (they usually are not). If you were expecting some sort of miracle there, I am sorry you were led to believe that. There are many, many websites that focus on IBS and you probably would get lots of ideas there. But, there are numerous folks here that also have IBS and manage OK. My husband has UC and IBS and does pretty well with Ensure and protein meal bars (they don't have that much fiber) when he is in the middle of a flare of IBS and/or UC.
You are probably thinking that if one more person says to be patient, you are going to explode. But, you won't. If you have a chill pill, now is a good time to take it. I know, stupid platitudes, but, it is all I've got, no magic fairy dust for you....sorry.
Jan