Point re: both FW possibly being charmed by Keyserlingk and later having suspicions even though he couldn't prove anything. The Wilhelmine comparison is a good one as well and given that even Kalckstein, who hadn't been responsible for Fritz for over a year and a half when the escape attempt happened, got accusing letters from FW afterwards, I can sure see FW holding a grudge towards Keyserlingk years later, even though he kept him on in his military position.

Another idea I just had: to check if FW had anything to say about the Rochow/Keyserlingk appointment (in early 1729 by the way) in his letters to Old Dessauer and lo! He had, except it's a bit incomprehensible - he reports (in March 1729) that he appointed "Rocho" and "Keiserling" and describes them as follows: "der eine ist cerios der ander salleter / alle beide [haben] verstand" I suspect cerios is supposed to mean "serious" but "salleter"? Even the footnotes are only guessing ("Komparativ of salé?") in this case. If it does mean something along the lines of "spicier [than Rochow]", maybe he did indeed mean to have one more serious and one more gaudy after all.

... oh, hey, and I see that in February, he was still looking for people. He's talking about Kalckstein's future position and then says "but not until I've found someone to be a 'bon amy' with my son". (Also: "Eurer Lieben [which is what he always calls Leopold] be so good and suggest a few people - if I choose one of them, good, if not, it'll stay between us, Kalckstein doesn't know anything yet".)

So it seems that he did intend for Fritz to like them and that he actually chose the right people for it in this case as well.