If any bits of the piston has got into the crankcase,( even a bit the size of a pin head ), as sure as eggs, are eggs, and no matter how gently you ride, there is bound to be carnage.
There is no way to remove all of the schrapnel from a crancase, other than by splitting it, and inspecting and cleaning.
No amount of flushing will remove a piece of grit trapped in a bearing.
No truer words could ever be recorded...I don't care who argues, bearings get replaced.