So this letter is genuine?
Yes. The formatting matches current and confirmed branch letters, and the letter is in the official ARC evidence (most likely provided by Watchtower themselves as evidence that branch Australia is doing their best)
I'm sure Watchtower would have jumped at the opportunity to show how apostates are liars if the ARC accepted a fake letter. What better opportunity to suggest that opposers make up all kinds of stuff, maybe even child abuse claims?