1) You use OW sprites. Using custom OW things that affect gameplay does NOT go against the "no gameplay affecting custom stuff" bonus point thing. You can still get the bonus points.
2) They affect gameplay, so yes, actually. There are ways to make other rooms, though.
3) You can have a max of two messages before you start losing points. Yoshi counts as a message.
4) Intro Message doesn't count as a message, but you get bonus points if you don't have to press a button during the intro.
5) Yes, but if they are major, they will count as custom things. If they are minor, they will not count as custom.
And yes, we should make a new thread. (although that's probably considered a dupe, so it's probably not allowed)