There are two options: Delete and Destroy.

Destroy is an option used on spammers. Their account is anonymized (the user's account information is all removed, including their name, email, website, about me, etc) and all of their posts are deleted. This can only be done to users with a very low reputation, unless you are a StackExchange employee. Therefore, if there is an urgent and valid reason for account destruction, and you have lots of rep, a mod cannot do this. You must make your case to team@stackoverflow.

Delete is more forgiving. When you delete an account, the account is anonymized (as above), but no posts are deleted. They just appear to come from some anonymous user who doesn't have an account anymore. Again, there is a rep cap for mods attempting to delete. If your account has too much rep, you have to go to team@stackoverflow.

Either way, you will no longer be able to log into your account. You lose all rep and badges. You will no longer own any of your posts.

To answer your other questions quickly,

4. Not unless you're a filthy dirty spammer.
5. Only if nothing of value is lost. This has to be evaluated on a case-by-case basis.
   6, 7) Only with the help of SE employees (more on this below)
6. Probably not without the help of SE employees

If you wish to erase your footprints, the only allowed method is to edit your posts to remove anything that hints at who you are, without destroying content. That means you have to replace what you remove with the same information, but with a different context that cannot be identified as coming from you. Failure to do this may result in your post being reverted and locked. You must do this prior to your account being deleted.

As for @you comments, those have to be removed by SE employees. I don't think any mod wants to spend hours doing something a query or two can handle.

All posts will appear to come from the same anonymous user. Only team members have the ability to alter this. You would have to request this via team@, but I'd give it a low chance of success.

There are two options: Delete and Destroy.

Destroy is an option used on spammers. Their account is anonymized (the user's account information is all removed, including their name, email, website, about me, etc) and all of their posts are deleted. This can only be done to users with a very low reputation, unless you are a Stack Exchange employee. Therefore, if there is an urgent and valid reason for account destruction, and you have lots of rep, a mod cannot do this. You must make your case to team@stackoverflow.

Delete is more forgiving. When you delete an account, the account is anonymized (as above), and only negatively-scored posts are deleted. All of your posts (deleted and not) just appear to come from some anonymous user who doesn't have an account anymore. Again, there is a rep cap for mods attempting to delete. If your account has too much rep, you have to go to team@stackoverflow.

Either way, you will no longer be able to log into your account. You lose all rep and badges. You will no longer own any of your posts.

To answer your other questions quickly,

4. Not unless you're a filthy dirty spammer.
5. Only if nothing of value is lost. This has to be evaluated on a case-by-case basis.
   6, 7) Only with the help of SE employees (more on this below)
6. Probably not without the help of SE employees

If you wish to erase your footprints, the only allowed method is to edit your posts to remove anything that hints at who you are, without destroying content. That means you have to replace what you remove with the same information, but with a different context that cannot be identified as coming from you. Failure to do this may result in your post being reverted and locked. You must do this prior to your account being deleted.

As for @you comments, those have to be removed by SE employees. I don't think any mod wants to spend hours doing something a query or two can handle.

All posts will appear to come from the same anonymous user. Only team members have the ability to alter this. You would have to request this via team@, but I'd give it a low chance of success.