DLA is difficult to get for IBS, but not impossible. Often when it is awarded it is at the low rate mobility because of bowel problems. Any care needs also are usually because of that aspect of the disease.
I do think it is wrong that the NHS won't prescribe homeopathic/herbal medicines, many of which are known to work. But that's a whole other political issue! You need to try to find a way to get extra help. Presumably you have a dietician? If not ask to be referred to one - explain your problems coping with diet needs. See if there is an IBS national organisation or a local group. Contact with other sufferers could provide useful information.
As for your question about what happens when you go onto income based ESA - it's just the same. You can't be penalised for what you spend it on.