I haven't any real suggestions for you at this point but I really do feel your plight. I have struggled for all my life to gain weight and since I developed anxiety & IBS it really has become a fight to even maintain what little I carry let alone gain any. Of course IBS really does complicate the problem further as it restricts what you are able to eat and how much. I think the first thing you need to do is work out which foods are triggering your IBS attacks ... sadly for me I've found that for me, most of those are the most fattening, weight gaining foods. And for IBS sufferers cheese can be one of the worst things! I know several years ago when I could still get away with it, cheese, eaten on toast at night helped me reach my highest weight ever!
Last year when I lost alot through a bereavement I started to eat porridge oats every day for either one meal or an extra and this did help me put on a few pounds ... until a few months ago I discovered I was allergic to it ... bringing my weight plummeting once again. And so back to the starting line .... bread is always good to bulk up on if you can eat that ....
This last week I have noticed a little but sudden rise in weight and I can only put it down to the potato cakes I am making to eat every day (mashed potato, mixed with either tuna & peas or a mixture of vegetables ... rounded and flattened in to cakes and then grilled ) ... I have 3 large ones per meal, and with bread and butter if I can manage it and so far that really seems to be doing the trick.
Aside from that I am also learning not to force large meals down me ... helping me need to go to the bathroom a little less.
It does take that old devil called time to go round the houses and figure out what works for you. After all, we are all different ... but I hope there is something in there that may be of some help, or give you a little hope.
Best of luck