If you have been to the Mayo and Australia for care, then it's unlikely I can come up with any insight that hasn't already been thought of.
To answer your questions, the antibiotic Xifaxin, or Rifaximin, can be effective in cases of irritable bowel syndrome. So the fact that it had some effect can be indicative of you having the condition.
There are other treatment options, including tricyclic antidepressants as well as antispasmodic agents.
Specifically to answer your questions, i) there is little scientific evidence to support leaky gut syndrome, and ii) I would pursue further GI consults, preferably another opinion at a major academic medical center.
These options can be discussed with your personal physician.
This answer is not intended as and does not substitute for medical advice - the information presented is for patient education only. Please see your personal physician for further evaluation of your individual case.
Kevin Pho, M.D.
KevinMD.com
Twitter.com/kevinmd