It is doubtful that a refrigerant type dehumidifier in an unheated space in Seattle would be able to work without the evaporator coil freezing up. Why don't you try the heater and see if the humidity comes down enough? Forget using absorbers; it would be like bailing out a boat with a teaspoon, you need to take gallons per day out of the air to make any difference.

I would not use any open flame heaters in a space with cars in it due to the explosive nature of gasoline vapors; they will travel along the floor for long distances. I'd even keep electric ones off of the floor to keep sparking of the thermostat contacts away from gas vapors.