Okay… revision to my first statement part #4 above. There IS high resistance to each solenoid when testing all the way from the controller. I misread the multimeter before. It was kOhm, not Ohms.
So, for example, testing with the multimeter in series between the control wire to the zone 1 wire, there’s a resistance of several hundred kOhms. Testing the same solenoid at the valve box out in the yard, shows 40 Ohms resistance. It is similar for other zones from the controller.
Resistance between zones (like connecting zone wire 1 and wire 2 across the multimeter) shows between 50 and 90 Ohms resistance for any.
Since it similar for all zones, I think this probably means something has gone wrong with the bundled control wires somewhere between the controller box and the first zone connected… guess I should dig up as far as I can and look for burns/melts/cuts?