It's interesting reading about harmonics and dynamic balancing, and the causes of the "clatter" of diesel engines. All of it new to me!
However, I would like to inject a note of practicality. Torsional impulses, harmonics and so on do not limit the life of the Diamond product, singles or twins. It didn't with the unsuccessful Thielert engine, and (so I understand) it doesn't with the more successful Austro engines. There are no prohibited RPM ranges either, unlike (for example) the old Tobago TB10, with a horizontally opposed four cylinder SI engine.
Re Diamond, you could tell it was a CI engine in two ways. 1) Power almost independent of RPM, and 2) a horrible clatter on shutdown, as the propeller clutch shuddered its way to a stop. That's it. (Ok, and the lightweight propeller.)
Don't mean to put a dampener on the discussion - I'm learning lots - but let's not say "they can't do that", when clearly, they can.