For anyone interested in my XU failure saga, through some testing on a side street, I've found a design fault... which perhaps might also be the cause of the problem that I reported at the start of the thread.
If, whilst operating the bike, I switch from ECO to SPORT or visa versa, it intermittently produces a 1-3 error code, which the manual indicates as a "High Throttle Disable".
If so, that label is a complete mischaracterization, as it seems to have nothing to do with the throttle, particularly high throttle.
I can make it happen whilst inching along at 1 MPH, as well as at 10 MPH, 15 MPH, 20 MPH.
Sometimes, it will clear before the bike completely coasts to a halt. Other times, the bikes roll to a stop and will not allow resumption for maybe 10 seconds or so. There also seems to be some sort of 'fully return throttle to the off position before re-enabling' logic involved in the timeout too, so perhaps that might explain the longer timeout I experienced days prior.
Before you say "just don't operate the switch", keep in mind that Zero used a really flimsy switch, and even if it wasn't so flimsy, it wouldn't surprise me that the slightest bump or pothole might cause the switch to momentarily bounce. That might explain the original event, since my hands were nowhere near the switch.
Electronics 101: de-bouncing switches
So, the MBB is likely part of the problem (since it re-interprets the throttle based on motor speed and the ECO/SPORT setting). The possible good news is that it could be part of the solution with a firmware change. Now, if only I can get them to take me seriously.