I have to laugh a little bit because I can hear my DD4 saying exactly that. Not in a mean way, just in a matter of fact way. She likes to get right to the point.
I would love to know the answer to your #1 question too. DS7 is usually pretty good about not throwing it in peoples faces, but every once in a while it comes out. He had a sleepover last night with a boy that he's been friends with since preschool. This boy is in PS 2nd grade. Well, the minute he walked in our door, DS wanted to show him long division and multiplying big numbers (356X785). His friend looked at him like he had two heads. DS quickly got the message, and they moved on to video games.
I just try to remind both of my kids to not say things that might hurt someone else's feelings, there is always new things to learn, and one person can't know everything.
