>So if user a deletes their comment and user b quotes them both by comment and name, should user a's name be masked when they delete said post?

I don't think so - it's clearly unreasonable to expect every instance of a username to be policed like that, particularly considering cut-and-pasting quotes. I'm fine with the idea that if it's in a thread it's no longer 'yours.'

But I also think it should be possible to remove ones' own account and have the username on those posts no longer point to an active account.

I also think there should be a "post anonymously" option as well, which there practically already is in the form of throwaway accounts. Might as well just make it official.