I know the resistive heater in my Volt can't compare to the heat put out by the ICE.

the ICE generates an insane amount of excess waste heat as a byproduct, so you have a virtually unlimited supply. The Volt is a PHEV so resistive heating was probably not considered super important.

If the heat pump can put out more heat for less energy

There's no if about it, it is ~300% more efficient.

That might be the second biggest issue (next to range)

They are the same issue. Less energy used for heating = more range.