Lesions aren't IBS, no matter what he says. The diagnosis of IBS is that there is no visible damage to or problems with the colon. And of course it is possible that you can have more than one thing wrong with you at once, so I think he shouldn't dismiss checking your gall bladder. I'm not familiar with the symptoms of lesions, but you do have a lot of gall bladder symptoms. Perhaps lesions in your upper colon mimic the symtoms of a bad gall bladder (a number of things do), but a hidascan is not a difficult test and if your problem--or at least part of it--is your gall bladder, then discovering that is better than letting you suffer, right? And if it comes back fine, then you can at least be fairly confident that your problem is the lesions and concentrate on treating them.
Hyoscamine is for cramping and it can be really hit or miss. Some people on here it works for, but for others, like me, we might as well drink a glass of water for all the good it does. It certainly never put a dent in my gall bladder attacks. But, there are a lot of antispasmodics out there, so if it doesn't work, definitely demand something else.
Personally, I wouldn't trust a doctor that dismisses lesions so casually and thinks that they are a sign of IBS. I think maybe you should find someone else.