It can’t be done, but if you have a very good reason to do so, contact a mod about it. If it’s just a preference thing though, I don’t think they’ll do it.
Yes. Click on “my account” at top of page, go to “delete my account” and click on it. There may be further instructions, I have never deleted my account before.
They’ll most likely change your username for you. I know of a few people who have gotten their usernames changed for one reason or another. Just be polite and ask nicely, I’m sure they’ll do it for you.