Yes, I've got nothing. I mean, Ziebura gives these four letters and the diary entry ten years later as her sources, so I assume that's all that exists. She does think Fritz maligned Marwitz, but her basis for assuming this is that she identifies Marwitz the page with Marwitz the quatermaster from the Rheinsberg obelisk, and states that surely, Heinrich, decades later after he was well out of love, would not have done Marwitz the honor of being listed with AW and other military guys if he'd been an STD-ridden cheat? But a) the Marwitz family was unfortunately large, and b) it's entirely possible that a guy gets STD and still is an honorable man who years later proves his valor in the 7 Years War.

(Fontane, who did see the obelisk in its original state - due to two WWs, it had to be renovated and today there's only the inscription about AW, with the original inscriptions to the other wronged-by-Fritz military guys on separate plaques, or so the German internet says, I haven't been to Rheinsberg yet myself - transscribes all the inscriptions in his "Rheinsberg" chapter, and there, the only thing mentioned is what I already included in another comment - that Marwitz the quatermaster made a good call at Hochkirch.)

Now, given Heinrich later that year wants to go on the Grand Tour (in a military educational kind of way, ahem) and is refused permission by Fritz who mentions in the letter that Heinrich didn't talk to him for six months and "sulked", I dare say Ziebura is on firmer ground in stating this year was when Heinrich's attitude towards Fritz shifted to "oh, how I hate you, let me count the ways" from whatever it was before. (Harder to say how much or little he ever had hero worship the way teenage AW definitely did.)

Since Lehndorff's diary entry is the only source that outright was Fritz was after Marwitz hismelf, it is of course possible that he was mistaken about this. Otoh, this sentence from the letters, put into the mouth of Marwitz-as-imagined-by-Fritz - "Doesn't everyone have to love me, adore me, worship me? What, you little villain, you resist? You haven't yet put your heart at my feet?" - makes me think Fritz was at the very least attracted and rather wanted to blame the page for that, no matter whether or not the guy actually as much as flirted.

Armchair psychology alert: I doubt he had had much to do with Heinrich while Heinrich was still a child. (Heinrich was four years old when Katte died!) So basically, this younger brother is still a stranger, until the second Silesian war, when they start spending time in each other's proximity. And lo, it turns out that of all his siblings, Heinrich is the one most like him; he's basically a younger self. Simultanously to this, Fritz has his big estrangement period with the sibling he's closest to and grew up with, who treasured that younger self but now in his eyes keeps prioritizing other people over him. And there's the fact that Heinrich, while respectful towards him, isn't prioritizing him,either; Heinrich's beloved sibling of choice is AW. So our ever paranoid anti hero who is now is proving once and all Dad was mistaken in fearing Fritz would party his inheritance away and in regarding him as soft in general now decides to double down on that younger self because of all of the above.

(And then there's always the possibility of pure sexual envy. I mean, however much or little sex drive Fritz had, neither Heinrich nor AW were given to platonic relationships, and did have a sex life. Von Krockow thinks there's a parallel between Fritz wanting to spoil young Heinrich's Marwitz romance, and be it with sarcasm, and much later in the letter after AW's disgrace where Fritz makes things even worse with his "the only thing you're fit to command is a seraglio of court ladies!".) Of course Fritz didn't want the court ladies, and he might not even have wanted Marwitz in particular, but someone was having sex and getting affection here, and it wasn't him.)