It'll only attract spam, so why would you want to do that?
This is "optional". You can hide your email if you want no one to see it but for those who want to put a public email address, this will be helpful.
Sure, but you know how people are. They see a field "email address" and they fill it in. Later, they wonder why they receive tons of spam.
I think 'display email address' option in 'Forum Profile Information'
[1] and a line about that option won't make confusion.
Yes, there's probably a more or less foolproof way to implement it. If you really want it, I'm not opposed, but I still don't see why.
BTW, what happens when you uncheck the box "Hide email address from public?" in your profile-account related settings? Isn't that what you wanted?