[FKHeath]

Two questions, does the 12v battery only charge as needed in HV Mode, and what should be the car off sitting voltage, as the dealer would not or could not answer that?

As others have said, the 12 volt battery is always maintained when the car is in ready mode.

Put it on Accessory mode, and about 5 minutes later I got the message shut off the vehicle to safe battery power.

That is to save the EV battery power, not the 12 volt battery. That is because when the car is stopped and the transmission is in park (or neutral) there is no way for the EV battery to be charged;

• Car is not moving, so EV battery charging by regeneration is not possible
• ICE cannot be connected o MG1 when in park, so the ICE cannot charge the EV battery.

This will also happen when you are waiting in line (fast food) or going through a car wash. EV battery cannot be charged so you get a warning about protecting the battery charge.