ok hypothetical situation.
i have domain 'willskewlstuff.com'. i have the email address 'email@example.com'. it's on the same machine as fbagroup.co.uk, and points to my username 'william'.
now if i send mail to 'wil', how does the machine know which username to send to?
since our mail machines have up to 8-12000 users, and up to 13000 domains, it's not really possible to do what you describe (at least not without a lot of changes on our end), while the usefulness to 99% of our users would be almost nil.
i do know what you mean tho - i'm very lazy and like to just type first names. my mail client will add the domain part to unqualified names that don't resolve to aliases; most email clients i've used in the past can do this as well.
no - you'd break your own mail. well to be more specific, our system won't let you add a subdomain with that name (or shouldn't)... if you wanted to do that you'd need to disable your mail service for that domain on our machines; then it would work. if you really cared about this, it would probably be possible to make it work, but difficult, and might cause you unforseen problems down the road.