THANKS AGAIN, EVERYONE! The antifreeze valve is now closed, and everything works great when on pump mode from FW tank
BTW, the warranty dept replied as to recommended max pressure regulator setting for city water supply (third party regulator on the end of my city water hookup hose) when not on FW tank/pump mode. They test the plumbing system and installed components at 80 psig water pressure, but cautioned that there are third party components (e.g. tankless water heater) that although having survived that test, may not be rated or warrantied for much more than 45 - 50 psi. I get pretty good flow in city water mode at an inlet pressure of 47 psig, so I'll just leave it there.