I applaud you for your efforts, you have obviously written off a lot of factors. From what your saying it clearly sounds like it's your stomach that is causing the acne. There's a reason for every thing that happens, and, don't mean that in a religous way. So, if you can't find a reason for why it is happening then you can't give in to the idea that it is incurable. Remember, Acne is only ever a symptom; fnding the cause to your stomach problem is what I would look for. You've obviously been doing exactly that, but, perhaps you need another way going about it. As far as naturopath's go, there are so many with different ways they can go about looking for problems and getting their diagnostic. I had been to four different ones before I found a lady who could acually diagnose me with a problem. Most of them do iridology (spelling??) where they look at your iris. That is always a good indicator of your body's health, however, it wasn't until I found a lady who does live blood testing did I find a problem that could be fixed. You acually look at your blood cell's on a screen with your naturopath and they can explain any problems going on in your body. I was surprised to find out that there are SOOO many factors with blood that can indicate bad health. They can then diagnose you with a problem going from the test. The way I see it, Acne is directly related to whats in your blood (if you count out external bacteria and what not).
I hope this is of some help. best of luck finding the problem and I know you'll get there with persistance.