If it's any comfort, Carlyle's reputation - well, not the general one, but the one of his Fritz biography as THE biography - has been going downhill througout the last century, even before most people detoxed on the Great Man of History dogma.

So I've gathered, but at one time, it was all I had, and my Katte-shipping heart now regrets that, grrrr. And I love your "detox" word choice!

it makes for a sense of getting emotionally left behind or at best having to share, and so her subconscious decided that first Keith, then Katte were suspicious characters

I agree that was a major reason. I think a secondary reason is that as Fritz got older, he started making more life choices that she couldn't agree with, and it was a lot easier to blame the people he was hanging out with than to blame him for his own bad judgment, or accept that they were starting to have different values and priorities. How often does "He/she's not a bad kid, he/she just hangs out with bad kids?" get invoked for teenagers throughout the world?

It also struck me that the two of them don't appear to have been close to any of their other siblings at any point.

I agree, I've always been struck by this. And...

It's basically a "you and me against the world" thing formed early on.

This, yes, exactly!

Those poor, poor kids.