I can't help you on the MRI question, but wanted to reply as I've had a lot of difficulties with anal/rectum problems. I have a very good colorectal surgeon, who gives me a rectal examination at every appointment. No prep or anything, he just does a digital examination and sometimes a camera (I'm not sure if it's actually a sigmoidoscopy that he does, as when I looked that test up it seems patients have to have an enema or laxative in preparation, which I don't, but I don't see what else his camera test could be).
I would really advise you to see a good colorectal specialist, if you can find one. Mine has always been able to tell me when I have hemmorhoids (internal and external) and rectal prolapses. I've had a lot of tears in the skin and skin tags, but not a full fissure. What I can tell you is that I've had bleeding and pain often but have never had a fissure, so your symptoms may be from other causes even if the red lines you saw are fissures.
As for the gluten and dairy problems, personally I'm wary of attributing symptoms to foods because when I tried to do that I had a tendency to see connections that aren't there. When you have constant symptoms, and symptoms that fluctuate all the time anyway, it's incredibly difficult to work out whether any of those fluctuations are due to something you ate, especially since you don't know if a reaction could be from what you just ate or from something you ate two days ago, or only occurs if you eat over a certain amount of a given food, or... you get the idea.
I'm not sure about the accuracy of coeliac tests - I think you can get a false negative result if you've not eaten gluten for a while before having the test. The test for lactose intolerance is fairly straight forward (and I've heard that you can develop lactose intolerance at any point, even if you were fine with it before). Unless you are consistently having obvious reactions to certain foods or types of foods, I'd try to get tests for coeliac and lactose intolerance done first rather than giving up foods.
What other tests have you had done? Usually investigating for Crohn's would involve a colonoscopy and upper endoscopy, so that would be where to start, but your symptoms don't sound all that typical of Crohn's, as typical Crohn's would be diarrhoea, bleeding, stomach pain, maybe weight loss or fevers, though of course some people have atypical presentations of Crohn's. It sounds like you need to find a doctor who's more on your wavelength and listens to your concerns in order to begin figuring out which tests would be best suited to finding the cause of your symptoms.
