Prevent unauthenticated users from using a registered users' credentials when commenting.
This will work on WordPress from version 2.1 and up.
Again, yes. I suggest putting it in
mu-plugins, but it works fine both ways.
Yes, but ... well. The only person I've caught is someone who is, apparently, pathalogically incapable of remembering that he HAS an account on the site, and he needs to LOG IN with said account. He also claims to forget his password and that WordPress doesn't email it to him, so I'm pretty much chucking his complaint as a problem with the user, and not the tool.