In your letter to them, you might want to write a series of numbered questions for them to address before writing out what you've learned from them ...
1. "Want to know where I got the VGCC antibodies?" I have no idea whether they think they're as easy to get as they think Mestinon is (getting Mestinon is no small feat), but it seems they'd believe even the most impossible dishonest or criminal act of someone around your age (whatever it is), if that person has difficulty breathing and swallowing. You want to know if they'd believe you could do that.
2. "Want to know why I didn't use them while I was there?" (A follow-up question tests their credulity a little more -- and I'm sure they'll respond "yes" after the first teaser if they take the bait at all
3. "Well, maybe I don't have a way to get the antibodies. Does that mean when I didn't test positive for them, I had what you said I had and then when I had the antibodies but the same symptoms, I had a new disease, that is, what the other doctors said I have?
So I have both psychosis and LEMS?
And when the antibodies don't show, I have whatever you said?
But when they do appear on tests, the same symptoms mean something else?"
And then, in anticipation of their response to number three, write out what you've learned:
If you ever get diabetes and you have a normal sugar (especially if you get normal sugar twice), you're never going to test for it again and you are going to stop taking your meds forever. Same thing for blood pressure. And same thing for cancer. Tell them that's what you've learned from them about how you are supposed to treat diseases you have. Ask if you learned the wrong lesson from your stay.
(Why that? Because you might imagine that they'll pull you off treatment and say you're controlled if someone sends you there again, you're terribly symptomatic and you get negative results.)
Frankly, I don't see much else to say to them. I'd only want to know what they'd respond to those taunts.
One more thing -- you'd specifically ask for the doctors' responses. Because you want to know what their "expertise" entails which completely defies common sense.
And I think you have to have written instructions somewhere not to route you there in case of emergency ...