I had an experience of which I was recently reminded that made the Locke/Demosthenes thing seem more plausible:
The Guardian Weekly has a page titled "Notes & Queries" where readers send in questions to which other readers send in responses. One week a question came up which I happened to know something about, so I wrote a moderately lengthy and well-written answer, with some solid facts, some plausible hypotheses, and a touch of humour, which got published in due course, with my real name and approximate address attached. A day or two after it was published, one of my housemates quoted something from it in conversation, and seemed surprised when I agreed. They were even more surprised when I suggested they check who wrote it and realised it was me. Since Locke and Demosthenes started out by doing exactly that - the equivalent of writing letters to newspapers that put ideas into people's heads without the people remembering exactly where they came from - their whole process suddenly seems more plausible. It's the equivalent of regular letter-writers to the Daily Mail and the Guardian who initially get most of their letters published, then get offered a regular column.
As for the anonymity thing, it's important to distinguish anonymity from pseudonymity - Locke and Demosthenes were well-established pseudonymous identities - like Mark Twain or Sting - "Locke" was no more anonymous than "Peter Wiggin" when he started, and probably much less so by the time he was negotiating peace treaties (though "brother of Ender Wiggin" would have messed with the relative anonymity of the two names).
As for the theme of the book, the point of using children and tricking them into genocide was explicitly stated - they needed someone who could understand the Buggers, get into their heads, understand and anticipate them, but OSC takes it as a given that when you understand someone that deeply, you can't knowingly murder them. They needed a child, not only because that level of empathetic insight is rare, but because they needed someone who could be fooled, who would accept the war as a simulation without questioning it deeply enough to realise that it was real. The whole thing was an explicit setup to trick a military genius into doing something monstrous.