Volt chief engineer Pamela Fletcher, in her video presentation on the 2011 Volt, said the concept of the Volt is to minimize the use of gas. That’s why they engineered Mountain Mode to recharge a fully depleted battery only to the SOC level that enables the car to maintain performance in high power demand driving conditions, rather than burning enough gas to fully recharge the battery.
In like manner, ERDTT and EMM functions don’t really require the Volt to shift fully into Charge Sustaining Mode, so the use of gas would be minimized if you continued to use grid power for propulsion. The function of ERDTT is to meet federal requirements for heating air to keep the window clear, and the engine doesn’t need to reach full operating temperature to accomplish that. The function of EMM is to lubricate the engine, not to maintain the generator. If the engine is not performing any "vehicle propulsion" tasks during ERDTT or EMM such as creating output torque for propulsion or generator cranking, then you don’t need to burn additional gas to heat the engine to full operating temperature.
So, during ERDTT, is your Volt being propelled by grid power or is the charge being sustained? Has anyone checked the energy usage display on the center console after driving several miles during an ERDTT episode to see whether or not the kWh Used increases as you drive, which would indicate the system is still using grid power for propulsion, even when the engine is providing hot air to keep the window clear, and even if distances are recorded as Gas Miles because the engine is burning gas for the heat? An EMM is over so quickly it might involve too short a driving distance to increase the kWh Used.
Of course, it would be difficult to determine the impact driving on grid power during ERDTT would have on overall statistics (Gas Miles driven using grid power but also while gas is being burned for non-propulsion purposes?), or if using Hold Mode instead would be more efficient overall. The immediate use of Hold Mode at the start of the commute would avoid using grid power for electric heat used before ERDTT kicks in (unless you preheat before departing?), but once ERDTT kicks in, grid power no longer needs to be used for cabin heat so the ev mileage would improve if you were driving on grid power...