Livre d'Or

that Gregor carries on perfectly trusting Miles, even in the direct aftermath of a massive betrayal

IKWYM. Although I actually didn't find it so: I think everyone knew Miles always had been unreliable but loyal, and probably always would be unreliable but loyal. Which is not a bad deal for the imperium, but that this was the kick that they had to draw a line and say "however much we love you, we cannot have completely maverick field commanders, or someone will get hurt even worse." However, I'd seen no reason not to trust him at a personal level.

a bit of a bad reaction from when I was a kid and awesome child protagonists ended up with boring sequels about what happened when the hero I was interested in turning into a dull, generic grownup.

*hugs* IKWYM. I don't want to trigger that nausea. However, I earnestly assure you that the remaining books are not "lets try to tack on some more" but books that I consider at least as much my favourites in the series as any of the others and as fundamental to my image of Miles' character.