Nick,
Thank you so much for that advice! I added the suggested digitalWrite line but it had no effect - the program keeps looping without any change in behavior.
So unless there is something blindingly obvious, I will shift over to a hardware diagnostic. FWIW, I have a saleae logic 8 on order, as the logic analyzer built into the QuantAsylum mixed scope is just too unintuitive to use for me (I obviously acknowledge that others may love it and find it super easy to work with).
One of the things I will send up to MCP for advice is the use of the clock crystal -maybe I am pushing it - but I didn't think it would have any impact on the SPI interface.