I have every sympathy for Joash, having been on a similar (but less soul-destroying) search myself.
I had 'something up' after my second child was born. My body just did not seem to 'click back into place' Could not lose weight, despite excercise, low GI, low GL, calorie-controlled intake etc etc etc. Saw 3 GP's, 2 endocrinoligists and a couple of other experts as well, thanks to Bupa I had every test under the sun (including hormone tests, sleep apnea tests, and tests for the more exotic things like cushings). Even went through horrible period on diabetes drugs at the off-label suggestion of one endo chap (because of syndrome X diagnosis) Eventually all I got for my troubles was a 'Firm' diagnosis of ME (my arse) and told to stop work immediately and start CBT counselling, and accept that I was really ill. Umm, no.
Anyway, I decided that if I couldn't get the experts to work out what was wrong I would do it myself. I went through all my tests and the only really unusual result (other than all the positive pointers to syndrome X, which I fully accept as being a factor) was that my retinol-binding-protein test was exceptionally (really exceptionally) low. It had been expected to be very high because this is the new fashionable way of gettign a confirmation of syndrome X. Then I took THIS info and tried to uncover what it might mean. Went deep into the medical research on RBP. There was nothing to link low RBP to weight issues as such, but there was sufficient to suggest that my body was possibly receiving plenty of vitimin A, but maybe not processing it, because of lack of RBP. I also found suggestion of Zinc issues. So I then embarked on religious approach to vitamins - Vit A, all the B's, Zinc and then multi-vits and primrose. I also undertook a series of liver detoxes as there was some evidence to suggest that sluggish liver can also reduce ability to process vitamins and minerals properly. All of this eventually kickstarted my body and now I am able to diet properly (although I still cannot eat anything like a normal person can - if for example I have just a simple dinner out, I need to spend the next week workign like the clappers diet-wise, in order to reverse the effect). But I am at least in charge. Still overweight but I know I can fix it in the usual way (cambrdige being my diet of choice although I find I have to stop it when I plateau, accept that a bit of weight will come back on, then re-start it after a break). None of this came from the doctors (and I have had to stop myself throttling the CD counsellor too, at times) - I just had to work out what 'my' resutls meant (and it took a lot of investigation). Any professional medic will accept a hit-rate of 90% say in terms of diagnosing patients so I learned to accept that no-one except me was going to fix me, because I wasn't in that 90%.
God, sorry, this is a long thread.
Anyway, I firmly believe that (1) the doctors don't yet know everything, (2) that for almost everyone (but NOT everyone) weight issues are easily resolved by diet/excercise solutions, that (3) if you have an underlying medical 'thing' (diagnosed or not) then things like cambridge may well work simply because they pump your body full of vitamins and minerals at same time as restricting weight (ie the vitamin bit might be doing the benefit more than the calorie/carb bit)
Have no idea if any of that gives you any ideas/support. I have no particularly strong views on gastric bands, though I did read somewhere that they don't work for everyone, and I have no reason to believe otherwise. I can fully appreciate why you want to go down this route.
Anyway, that's my experience. If you were a mate in RL I would strongly advise you get ALL markers tested (including retinol-binding protein) and I would also advise slavish devotion to vits/mins, and the detox stuff (google epsom salts/liver detox for directions on how to do this). In all probability your genetic make up and mine will be miles apart so none of this will help. But it might.
Good luck in whatever you decide to do.