If your only problem with the prescription coverage is pre-authorization (they just need a form filled out by the doctor once a year) then, yes -- once things get rolling you shouldn't have a problem. If you have really low maximums and there is not coverage for the drugs then, that is a different story. Keep on them and keep asking for an explanation of why they aren't being covered.... again if you need help with things like that, ask someone at the clinic... either the nurse or social worker.
We don't have to deal with a lot of supplements and enzymes -- but from what I've read on other posts over the years, people do not generally get the insurance companies to pay for anything that is not prescription.... I really do not think you will get them to even consider Carnation drinks. I do think that some people keep track of or are able to use all the additional medical and food expenses as a tax right off though.