[Earth]

I think it should be invite-only, BUT: there should be a submission form for users who want to join.
A simple message, that will easily determine the quality of the user.
And then a simple disablement put on them: they can only post (no negative repping) for the first ~5 posts which are then reviewed (all at once) and chosen whether they stay or go.
So basically, the reviewing of newbie's post's could be spread out among the more trusted, established (HQ) users.
I don't know about anyone else, but I would be happy to do atleast one review a day (if I qualify as a "trusted, established (HQ) user" ).

Maybe I'm just dillusional.. but I think it's a fair compromise.

I think it would be more simple to just manually approve them members, make it so all new members go into awaiting activation but cannot post or anything, and only allow them to be manually activated after the account is looked over and cleared of being a possible multi-accounter then go ahead and activate it.

Simple but could be effective.

Your method is still good but it would require some sort of plugin or constant monitoring of emails to approve them which could become a hassle.