The Ender's Game parallels are definitely striking, and I like them. That was a fun book, and I am enjoying seeing it played with by an author so bluntly opposed to the "whiny hero" trope.

I'm somewhat surprised that Dumbledor didn't intervene, say with a side quest, to bind some friends and allies to Harry.

That may have been part of the purpose of Dumbledore pushing Hermione into being a heroine: he wanted Harry to have an equal partner instead of being isolated at the top.