: been here. I've just been out of frame." It feels like a huge retcon.

There you go, you've got it. There's been a few of those. It's like game of thrones- if all you know is the TV show, then of course you get surprised by the sudden changes and nuances that the book readers are already aware of. That's probably the best analogy I can think of. Book canon vs game canon vs head canon vs gameplay for the sake of gameplay.

: If these characters are so important to Chief, so important to the UNSC and
: humanity, and were alive for the events of Halo CE - Halo 4, why has no
: one even whispered their names?

Again, because you can only cram so much into a game before it becomes a movie and then at that point why even bother anymore?