Here’s the electrical drawing of the area of interest - freezing hands and slow flowing ink in the hangar today.
Spending the morning tracing and troubleshooting. As per standard “could it duplicate” today. Or rather, the problem is manifesting differently.
Resting battery, nothing on today is 12.4 V. Oddessy PC680. Lower than it should be, especially right off the battery tender. Clue 1. Checked voltages all along the path in the picture. Nothing unexpected.
I can turn on the master (via either switch now that I see how it’s meched - interesting redundancy and a lesson me (non-builder) to better understand the systems!). I can turn on all the electrical systems. Voltages readings aroud 11.8 now.
On to normal start up sequence, engage the starter and the entire electrical system shuts down.
I connect it to a good battery external and now everything works.
Sometimes it is the simplest solution - I sure went down an odd path because I was biting off on the voltage immediately after removing the battery tender then didn’t check again later.
Thanks for the suggestions and encouragement to better learn my system.