I'm glad I could help (even a little!). I've lost about 20kg so far and I'm hoping to lose about another 20 or 30. And I sometimes have to eat nothing but white bread, dry biscuits and milk for days and I've still lost weight, so don't listen to people who say you can't lose weight if you're eating simple carbs. In an ideal world, it would be healthier to always eat whole grains, but it's more unhealthy to be not eating at all for three weeks! Also, given how severe your IBS sounds, it could be worth seeing another doctor for a second opinion and getting some tests done to rule out things like Crohns Disease, nodules in your bowel or parasites (unless of course, you've already had all that done). It'd be a good idea to check how much Zantac you can take too, I find that doubling my dose for the first few days always helps.
Ok, all that said, here are my rules for cutting down on carbs without killing my insides:
1. Go slow. I start by having a carb free dinner, then snacks, then lunch. Almost never do I stop having my delicious carby breakfast. Also, don't be too stubborn - if bread with dinner one night will mean you won't be in horrible pain tomorrow, then it's not a big deal.
2. Don't get too pedantic. I don't think pumpkin is as bad for weight loss as some think it is. I love a nice roast pumpkin and spinach salad and it has way less calories than a chicken and salad roll. If your version of a low carb diet involves sweet potatoes and pumpkin, then so be it. Potatoes are really high GI so I do tend to avoid them but they're still full of vitamins and minerals, so don't stress if they creep in.
3. Keep things bland(ish). I know lots of people go crazy for fancy salads and marinades and these are great sometimes. But I really find that to keep my stomach in good condition, it's better to just keep it simple most of the time. Most days for lunch I'll have some home made soup or a salad with a tiny bit of dressing, fruit and low fat yoghurt for snacks and a small piece of meat and some cooked veggies, like asparagus or green beans, for dinner.
Ok, so this is all I can think of right now! No doubt I'll think of things I've missed, I'll post them here when I do