Mixing Valve Questions

A mixing valve is much like a automobile thermostat. A temperature sensitive spring holds a valve closed, as the set point is reached the spring shortens and the valve opens. In a mixing valve the hot port is thru and the cold port is off. As the temperature increases in the hot port the spring begins to shrink allowing the cold port access to the output.

Taco 5000 specifications:
Maximum differential pressure between hot and cold 10:1
Minimum pressure 1.5 psi

In your questions you want to know if the hot port can overdrive the cold port. Since the two ports are connected together during mixing it would seem that the port with the higher pressure would drive into the lowest pressure port, but my guess is that the mix port is the lowest pressure port. So my guess is it should work as long as the mixing port is the lowest pressure (suction side of pump).

If the pumps are off the heat from the hot water can only move by conduction, so you second question should be that the hot water will not leak into the basement zones.

I have a similar setup and I get some hot water bypassing into the cold pipe but very little circulating through the zones when the circulator is off.

I have a Caleffi mixing valve on the output of my DHW tank and I had to install a check valve on the cold side to prevent reverse flow of hot water into the cold side. Once the spring is hot, flow can move from the hot side to the cold side. A swing check on the cold side will prevent this without introducing much additional head pressure.