I really enjoyed the notebok entries. It made me feel like I was there!
I don't know whothe airplane guy might have been but I am aware that he conducted a long correspondence with a Canadian called Bill Lishman, who flew a revolutionary bi-wing hang glider called an Easy Riser, which he had motorised with a go-cart engine in 1978. Bill had became a leading figure in the development of ultralights, and he was ultimately famous around the world as Father Goose after he reared geese and taught them to migrate by leading the way in just such an aircraft.
He did so after purchasing a number of motors from Dieter so that the first geese to learn to migrate alongside an ultralight did so to the guttural growl of a König engine.
Of course your airplane guy predates this by some years.
I do know that Dieter was interested in hang gliders from around 72 - 73.
The accident was indeed a tragedy and seems to have been sadly predictable.