There is no right way to fight this because you're talking about a legal argument when really the issue is the facts.
Every jury is different and if you refuse Article 15 they will court martial you. As I said, they completely trust these drug tests (commanders). There are certainly times when juries choose not to convict when a person says that they didn't do it, but that's very rare and, as you can imagine, there is no rhyme or reason to why they choose not to conviction.
Certainly, when we are talking about a "naked UA" meaning that we have no admission of guilt or other evidence of the use, a good military character defense has been shown to be the most effective method of defending against this sort of claim. That being said, it is a risk to demand trial by court martial. If you end up being in that group of individuals, where they jury relied on the UA no matter what you say, then you'll end up with a federal conviction for drug use, which would follow you around for life.
I know that you want a legal strategy here, but there isn't one. They already have the positive drug test. That is all the evidence that they need to legally move forward with ArticleXXXXXmartial, and an administrative discharge.
You are stuck either proving you were given the wrong medication or trying to challenge the validity of the test (which again, works sometimes and doesn't work sometimes, no rhyme or reason).
So, you can accept the Article 15 and make your arguments there, hoping that your command listens. You can demand court martial and ask for a jury (don't go judge alone, because they aren't as suspicious of drug testing as most people can be) and hope that you convince them.
I'm sorry, but you really are in the place where all you can do is ask people to believe you and hope that they do.