The thermostat bypasses the radiator when cold, but not the engine. The coolant has to be allowed to flow in order for the hot coolant to fully open the thermostat. Being electronically controlled means there just needs to be a sensor near a known hot spot to trigger flow from the pump.

I’m not familiar with the impeller shroud you mentioned, but I looked it up and the description seems to agree: “This pump includes the shroud and control valve to restrict flow while the engine heats up.”

Whether or not it affects the time required for the heater core to be operational would depend on how they decided to route it, and if the solenoid offers variable positioning. I imagine it is variable, otherwise they’d create thermal shock every time the engine heated up and the pump suddenly started flowing colder coolant through the block, so technically it should be possible to fully replicate the general functioning of the thermostat and heater core. Now that I think about it, it’s most certainly variable and it’s why they didn’t go with a clutch system.