One could debate whose fault this and that was, but the bottom line is that these engines were not properly tested before hitting the market.
What Diamond+Thielert should have done is build say 20 planes, and give them away to various types of users around the world. Some high time, some low time. In fact Thielert could have fitted loads of Cessnas and Pipers with the engines and just given them away, in return for regular feedback.
That's what I do in my business (electronics). The cost of giving away a load of stuff is a miniscule fraction of the cost of fixing real stuff shipped to customers.
Why did Diamond/Thielert not do this? I would suggest because aviation is a business where you can "do a Lycoming" (talking about their cranks, notably SB569 but all the preceeding stuff too) and get away with it.
Under European consumer legislation these owners could sue Diamond into the ground, and no doubt some have done (D had to give 100% refunds in some cases I know of, on well used planes) but business users (e.g. schools) can't because the consumer protection legislation doesn't (usually) cover them. In reality, most owners won't sue because a) people have better things to do and b) you will then get NO support from the dealer, the factory, absolutely everybody will wash their hands of you, and you are stuck with the plane......