As you suggest, I was somewhat worried that I had been trying too many combinations and had gotten confused about the state of my two machines. On January 8, I attempted to reset both back to a clean state and current source, then retried the combinations (Go builder CI versus /usr/src make -j24, sysupgrade -s versus locally compiled without WITNESS versus locally compiled with WITNESS). No more crashes seen yet.
Many thanks to all of you!
