Exactly, and while I'm sure he had decent examples in the sense that he'd received at least some decent condolence letters in his life, I don't think he understood the principles of what makes a good condolence letter, especially the part where holding back on saying everything he thought would be a better move in this case than giving advice.

He was, after all, an inveterate advice-giver (see also: teaching castrati to sing), and I don't think he had the tools in his emotional toolbox to figure out that while this might be the advice he gives himself at a time like this, it's not what he needs to be giving Heinrich if he wants to help him. That's the part of the letter that's easiest for me to forgive. (And by "forgive" I mean "it is specifically the fact that this letter is so terrible that means it requires forgiveness.")