There are probably dozens if not hundreds at Bethel who were aware of the change by the time the leak happened so it doesn't have to be someone of high status who leaked the information.
While I'm sure you're correct that many at bethel would've known, is it true that this letter could've come from any one of them? The leak wasn't that the TMS was going away or that the magazines were getting downsized again, it was the entire letter, verbatim. I wouldn't think that everyone at bethel that's aware of the changes would have access to the letter necessarily. That said I'm privileged to have never worked at bethel so I could be completely wrong on this.
My theory is that the leak came from someone in the IT department. They've probably got folks in IT that haven't been in very long, and a background in computers is probably something that primes people for waking up (more likely to explore doubts anonymously online). It would make sense that they'd want to get the letter ready to post ahead of time (especially if it's been translated and is going to link to a video, etc) and this would be a much more likely (to me) candidate for the source of the leak than someone on the service committee or something like that. Another possibility is that it could've come from someone in translation, as they sometimes take people that haven't been in the cult long due to their qualifications, from what I understand.
I would consider a good 'leak' to be something that can potentially be damaging to them. Just leaking information ahead of the official scheduled release date or 'elders only' leaks can help 'if' they expose them for something illegal, unethical, or not biblical. Otherwise it's just getting information ahead of time.
IMO every leak is a valuable leak. Just proving time and again that their own elders and bethelites aren't as devoted as they'd like everyone to think is worth something in and of itself. Even if that only means that we get information a little early.