there's what I would argue is a clearly pathologically high false-positive rate for detecting "Dumb/Not worth my time".

Do you, by any chance, have any data to support that? I am sure there are people for whom it's a problem, I'm not sure it's true in general, even among the nerdy cluster.

If you ever have the idealized ceteris paribus form of the "I may only talk to one of two people, I have no solid information on either" problem

That's a very common situation at parties where you circulate among a bunch of unknown to you people.

-"Thinking clearly," as in "not making reasoning mistakes I can immediately identify?"

Nope, that is thinking correctly. Clear thinking is a bit difficult to put into words, it's more of a "I know it when I see it" thing. Maybe define it as tactical awareness of one's statements (or thoughts) -- being easily able to see the implications, consequences, contradictions, reinforcing connections, etc. of the claim that you're making?

I would claim that humans, myself included, are notoriously bad at assessing it, generalizing it, or for that matter agreeing on what it means

I don't think I would agree. Making fine distinctions, maybe, but in a sufficiently diverse set there is rarely any confusion as to who's in the left tail and who's in the right tail. And I found that my perceptions of how smart people are correlate well with IQ proxies (like SAT scores).