do we think Lehndorff is onto something there, or is he projecting

I honestly think he's projecting, between the letters being affectionate right up to the end, as you say, and also just how *long* Fredersdorf had been sick and unable to accompany Fritz on his journeys. I also gathered from Preuss, but could be wrong, that there was another batman in between Fredersdorf and Glasow. I would need to read more closely, but I think the sequence may have been "Fredersdorf got sick, someone named Anderson fell out of favor, Glasow got his turn."

Then again, as Mildred pointed out, emotions (and jealousies) aren't logical.

They're not, but they're often consistent, and if Fredersdorf didn't care about Marwitz or any of the other witty pretties Fritz collected, why would he step down for Glasow? I don't think Glasow was *that* special.

I vote for Lehndorff projecting.

Of special interest - which came first, Fredersdorf's marriage plans or Fritz showing an interest in Glasow?

I would need to check. My guess is Fredersdorf's marriage plans, but I could be wrong.