I mean, I mostly thought Carlyle's problem (having only read these bits out of context) was that he depended rather too slavishly on Wilhelmine as a source without thinking that she might have had her own (psychological) agendas.

You can see him slavishly depending on her! He might as well have "Wilhelmine Is My Source" blazoned on every page. I'm also not sure, even from his Wikipedia page (maybe selenak knows more, she often does), how comfortable he was with Fritz's sexuality, which tends to seriously color how people present his favorites.

[ETA: I should also note that Carlyle may have shared Wilhelmine's intense subconscious desire to distribute as much blame as possible to Fritz's companions instead of Fritz, because Carlyle's whole philosophy toward life, as exemplified by but not limited to his bio of Fritz, was "Great Men Rule (Both Figuratively and Literally)." Fritz was a great man and ruler; therefore we need to explain away the things we don't like as much as possible. So I suspect Wilhelmine's take on things really resonated with Carlyle.]

I'd probably have been just like Carlyle and taken her at her word

It's easy to do! I myself, every time I went to Carlyle looking for shipping material, came away with, "Well, maybe I'm over-romanticizing the whole thing and young Fritz actually really had bad judgment about boyfriends at that age. There's no reason he couldn't have, after all. I still *wish* I could have a proper ship for them, though!" It was only after examining multiple sources recently, some of which called attention to the biases of others, that I managed to arrive at my own opinions (and awareness of where the boundaries of my knowledge lie).