The cassette path appears to have been solved:
1) Replacement of pickup roller assembly permitted sheets to leave the tray, only to result in paper jams further down the line on the "deck side" of the path (meaning the actual printing plane). We reasoned that jams were occurring as a consequence of media deformation at the leading edge.
2) We imagined the cassette tray's rear side was not lowering properly so the entire tray was replaced with a new part. The tray replacement, and the new pickup roller, has this unit working correctly wit 100% feed and 0% jams. We can't comment about long term results, but we can say that 25 sheets have fed, and several weights of media have been tried. This includes spec and "non-spec" papers like 310 weight Platine, Museo Rag and Baryta Photographique. My first 4900 fed more than 2000 sheets of those papers without any problems, so I'm hopeful this will too.
In summary, I think the tray's "rear door" was at fault from the start and the actual pickup roller was not. Otherwise you'd have to blame the initial failure on two unrelated parts.
John Caldwell