Thank you LouNY (so no one pronounces "LOONY" do they? :)), thank you also Henro & Ed. I don't understand the relay logic either.
I hot-wired the normally closed fuel solenoid, took a butterknife to the battery hot + solenoid, key on, and it started up.
There was, however a...
It's Memorial Day, 2022, tractor is dead in the vineyard.
What I learned:
1) Both seat sensors are working (Side lesson; seat tip-up sensor is so you can operate rear PTO WITHOUT your butt in the seat and tipping it back will shut off the unit),
2) Typical fuel solenoid is 2 wire, 12v HOT to...