I can sympathise on the MIL front - when we take Honey to my in-laws she is desperate to give her all sorts of treats!!! Honey doesn't have any problems tolerating foods, so that isn't too much of an issue, but obviously we don't want her eating all kinds of rubbish!
Fortunately MIL is willing to listen to some "groundrules" - we do allow Honey to be a bit spoiled there, but not if it interferes with her diet. She will get any peices of veg when being prepared, but other treats have to wait until she has eaten her meal. I cut down her meal sizes a little so she doesn't put on any weight and we reduce her normal treats too.... Try working with your MIL - as stuffster said, see what treats you can give Merry and give your MIL a big bag of them when you visit, so she can still spoil him. Assuming he can tolerate oats, get her to make him his own porridge (made with no salt and only water) - I have given this to Honey when she was feeling unwell. If this doesn't work, get your OH to step in and do the arguing!!!
Good luck finding what triggers Merry's colitis -