Wow, these are all great theories!

I looked up Newgate; it's actually in London, so it could easily be her. I really like your idea of a canon nod, though. Mycroft Holmes is supposed to be equally as brilliant as Sherlock himself, so I was a little confused by his portrayal as a more ordinary person; this would make sense. He would be able to cover himself up so well that not even Sherlock would suspect anything.

When I first saw 'M. Holmes', my first thought was that it was Mycroft, so that is very possible, too.