...and then you tell them that it's exactly like someone writing a fake return address on a piece of mail before sending it to you, and they understand, because as stated above, this is conceptually very simple. You don't need to talk about "headers" at all - I don't know why you're bringing them up.

Exactly. And now imagine you manage in some way to explain average user that he cannot trust email sender. Then explain him that if email is signed then it is OK. He will most probably answer "why the hell now should I trust email sender !? There is just a dumb icon saying the email is signed. I do not trust it !!!". Which would just be the most sane reaction actually