Success! Guzzi back on the road. Here's the data in case it helps someone else.
I replaced the original phase sensor with a new one, along with a new O-ring. Since I had the tank off, I replaced the original half plastic/half metal fuel filter. Also replaced the downstream fuel line I managed to break while removing the tank.
Go to restart. Now the starter *would* crank for a solid 3-5 seconds as it always did before. So I'm convinced that something in the system cuts off juice to the starter motor after 1/2 to 1 seconds if it detects the absence of valid phase sensor data. With the new phase sensor installed, it was getting input, and thus permitted longer cranking. This has nothing to do with the battery, starter, solenoid, etc since I didn't change any of that stuff.
I did 4 crank attempts for 3-5 seconds each time. Eventually all the air was purged out of the fuel lines and the engine started.
And I can confirm that a '13 Stone has no shims beneath the phase sensor. It's just an O-ring.
