LU
“I think we’re talking about a case where you have reason to suspect that you can’t trust the person’s kashrus.
Or that you don’t have a reason to assume that you can.”
These are 2 contradictory statements. The rest of your post is similarly riddled with the same mistake.
eid echad means you can eat other people’s food unless you have a reason not to. “The average Jew in America is not even Frum.” well that is a reason not to. So the line that follows doesnt make sense.