I was referring to exclusively the examples here. (My response edited out the quote, so it wasn't clear). All three of these are chazal situations and I find it hard to believe עד אחד doesn't apply.
Agree to the first
ETA not really because if his religious status is unknown and he’s selling it we pasken EA won’t work iirc.
To number 2, you think it can be used on pesach? Also, did he bake the matza? Maybe he is saying it’s kosher because it had a hechsher which you personally don’t trust. If you’re makpid on yoshon there is no need to ask if he meant that too?
To number 3, if it’s a machlokes about tolaim that you’re personally concerned for how do you know he he is aware or concerned for that opinion? Also, did he personally check for scales? Maybe he got it from a hecsher he isn’t makpid on Rav Moshe.