[martae]

If price was no option, I'd say give it a better battery. Maybe you could find a wrecked electric, and put another battery in parallel.
Offer to run another conductor to the electrical outlet you use at work, and make it 240 VAC. Materials wouldn't cost much. If it's in conduit it would be very easy. You could put a gang box on it, keep 120, and add 240 beside it.
Use 0W30 Mobil 1 in the engine, and put Tufoil in it. I use it, and it seems to work. If the Volt has any sort of gear box or differential, use the lightest multigrade oil you can, and add tufoil.
Try Bosch spark plugs with the four electrodes on the sides, I think they call them "crossfire". They increase engine efficiency by allowing better flame propagation. You could use less spark advance, since the electrodes run a bit cooler, predetonation is a bit less likely. You could also run a bit leaner fuel air ratio, since they give better ignition.
The manufacturers of hydrocarbon refrigerant replacements claim they increase the thermal efficiencies of heat pumps(air conditioners). I tend to believe them, since a lower mass of refrigerant is used. People will tell you they are dangerous, but the refrigerants are much less dangerous than the gasoline already in the car.
Use some sort of baffle, or louvers to modulate air entry through the grill based on whether the AC is being used, the engine is running, or the weather is cold. Not only will this improve aerodynamics, you'll have faster warm ups, and the engine and batteries will stay warmer when parked. Battery performance is related to temperature, try to find what you can about your battery.
According to Wikipedia, the second generation volt(beginning model year 2015) has a better battery, with different chemistry. Find if they are the same size, and shape, and keep an eye out for a totaled one.
Based on WikiP, the battery is made of individual units, that should be able to be swapped out. You may be able to swap some, most or all your units for the new ones. Presumably, the cells are all in series, so you can mix them, if any are in parallel, you must keep the same chemistries in parallel. Since battery voltage changes with temperature, and age, there will be flexibility built into the inverter(s). Some parameters might need to be changed in the computer that monitors the battery.
Try to come up with a way to dump the engine waste heat into the passenger compartment in the winter, rather than using resistance heaters. I think the electric motor is liquid cooled, the inverter may be too, so a small fan/coil unit with associated piping. You may be able to use the ones for liquid cooling computers. There will already be a pump in the system.
LED lighting wherever possible.