All of those scenarios you listed in your first paragraph could be true, but I, by definition, do not have that information. If I do, it's because my senses tell me so. I can only reason from the information I have, after all. I'm assuming everything is accurate because there is no other assumption I can make. It's not merely a convenience, it's a logical necessity.
As for the second, sure, you COULD observe them, but I can't know whether you can or not. All I can reason from is my own observations, which are sufficient to tell me you are probably not a unicorn. For one thing, your typing is significantly superior to what I'd expect from an entity typing with a horn. But that aside, if you do nothing to show that you're a divine being, then I can't observe any evidence that a divine being exists. I can't grant you that you are a divine unicorn for the sake of argument, because that's the very thing we're arguing about.
Now, I'm not trying to insult you, but you did start by stating that agnostics logically ought to simply be atheists because disbelief is equivalent to nonbelief, which is something of an insult to someone who follows that principle. And there's no inconsistency with having beliefs about nonbelief, either. Metabeliefs are totally permissible to an agnostic, just as it's possible for somebody to be intolerant of intolerance.
Finally, if they believe it because they were taught it, fine. That's their choice; it's when they start to teach others or convince others to convert to their beliefs that there's a problem. I don't care what somebody believes, I just care when they start trying to control what other people believe.