I agree with everything you said before the last paragraph.

But here's why I continue to raise this point: I'm in a position where many people who work on FAI full time -- who I have heretofore considered FAI experts, or as close to such as exists currently

Just because somebody works on a subject full time and claim expertise on it, it doesn't mean that they necessarily have any real expertise. Think of theologicians as the textbook example.

Either (1) I am wrong and stupid; or (2) they are wrong and stupid.

I think this dichotomy is too strong.

People can be smart and still suffer from groupthink when there are significant social incentives to conform. MIRI technical work hasn't been recognized so far by anyone in the academic community who does research in these specific topics and is independent from them.
Also, MIRI is not a random sample of high-IQ people. Membership in MIRI is largely due to self-selection based on things which include beliefs strongly correlated with the stuff they work on (e.g. "Lobian obstacles", etc.)

So we have the observation that MIRI endorses counterintuitive beliefs about certain mathematical constructions and generally fails at persuading expert outsiders despite their detailed technical arguments.
Explanation (1) is that these counterintuitive beliefs are true but MIRI people are poor at communicating them, explanation (2) is that these beliefs are false and MIRI people believe them because of groupthink and/or prior biases and errors that were selected by their self-selection group formation process.

I believe that (2) is more likely, but even if (1) is actually true it is useful to challenge MIRI until they can come up with a strong understandable argument.
My suggestion to them is to try to publish in peer reviewed conferences or preferably journals. Interaction with referees will likely settle the question.

Even after all of the above argument, on the outside view I consider 1 more likely, but no one has been able to rule out 2 to my satisfaction yet and it's making me nervous. I'd much rather find myself in a world where I've made a silly error about strong AI than one where MIRI researchers have overlooked something obvious.

Beware wishful thinking :)