Whatever the explanation, I'm the only one to blame. And yes, anything's possible- I got the endplay wrong, face of the sleeve wasn't perfectly flat, or a dozen other things. Until this failure, I hadn't given much thought to the design of this area. But studying it now I'm puzzled about why two steel surfaces are constantly running against each other at near engine speed. Other similar areas of the hydro typically use bronze-against-steel. That's the first part of the puzzle. The second part is- why does this design usually work fine, but failed this one time.
There's little chance of finding someone to shed light on this, so I'll probably have no choice but to replace the parts and cross my fingers. But... god, how I hate pulling/replacing this thing...