Eliezer changed the original text of the first chapter a fair bit...

Ah, you are right. I was looking at an old .pdf copy when I accused you of excessive abridging. As for abridging the flash-forwards, you probably made the right choice. Though you could present them before the announcement of chapter name and title and other boilerplate.

As for the echo chamber, I would save it for things done in 'Magical Voice' - spell invocations, oracular pronouncements, sorting hat, etc. I do agree that the letter-writing and thoughts-to-self ought to be distinguished in some way, and perhaps in the same way, but the echo/reverb just doesn't seem right for them. They should somehow be less portentious, rather than more.