I think the author may have been Irish, because perhaps a person might write as they spoke - seems quite logical. I think it's slight evidence though, and not at all conclusive.
Ginger Beer bottles not withstanding, I don't see how this would implicate, say, Barnett at all though, since having been born and bred in London I think there's a fairly good chance he would have sounded local.
Depends. Cultural identity's a tricky thing to quantify. The Lusk letter is disturbing. I personally feel it could have come from Jtr. It's possible.