I have not found anything from an authoritative source that can completely answer your questions. However, Prof. Whorwell at Manchester and others are beginning to address such issues using brain imaging techniques. He discusses some rudimentary thoughts in the paper referenced below. Here are my thoughts.
Lactose & fructose are FODMAPs. Lactose intolerance and fructose malabsorption have known organic causes. Hypnosis will not cure either.
A characteristic of IBS is hypersensitivity to GI stimuli. FODMAPs can produce stimuli that exacerbate IBS symptoms e.g. gas. Hypnosis can help desensitize one's sensitivity to GI stimuli. Depending on the degree of sensitivity one has to FODMAP or other GI stimuli it may be possible for you to become less sensitive and hence less bothered. This does not mean the underlying organic processes involved with FODMAPs or any other GI stimuli are gone. You have modified your perception and other physiological processes.