FYI
I have been in the business and just some comments
Take the advice and get off automatic delivery. Just watch you don’t run out
Electric heaters are inefficient; ever see an energy star rating on one? The losses are all at the generation station. An oil heater is at best is 95% but most are 80%. Gas fired are the most efficient with rating up to 97%. Like MPG, higher number is cheaper to run. Run your make and model number on the internet and look for its AFUE.
The reason you vents are below windows is to “wash” the window with heat. Besides the heat losses from windows the point is to stop condensation from forming on the windows. Next time you visit a house with radiators look where they are. Below windows.
You could mark the position of the vents with a sharpie and then close down the ones in the rooms you don’t use. This way if you want to heat the whole house you can return them to the original setting and will not mess with the balancing of the system.
Take the advice and check your filter. It will be on the side of the furnace. That is how air gets in and is heated and blown to the rest of the house.
I think your biggest problem is the location of the furnace, You said it is in the cold basement about 45 deg. Moving it is out of the question, running duct work for the return is probably impracticable. Insulate your basement and the duct work to the house. Think about it, every time your burner comes on it has to heat 45 deg air to 70 deg to warm your house. That takes lots of energy rather than 68 to 70. I’ll bet you would still save gas if you moved one of your electric heaters to an insulated basement.
I would not spend the money on blown in insulation on the walls. It has a tendency to settle to the bottom after a few years. I would go to Home Depot and buy rolls of insulation for the attic if it has none or very little. It is easy to just roll it out.
All that being said it probably does pay to only heat 500 feet of a 1900 square foot house by electric.
I would stay on autofill but make sure to prebuy enough gas to get thru the winter. The advantage of autofill is if you run out somebody has to get out of bed and fill your tank at night. Price wise autofill normally is cheaper. If you prebuy your gas it always autofill unless there is a locked gate or something that keeps the driver from delievering. The advantage of owning your tank is the ability to use different companies. The disavantage of owning your tank is if something fails on it its your problem. By far the best service propane companies offer is the prebuy plans.
The space heaters are virtually 100% efficient at putting energy I pay for into the room, with no duct loss. And here the energy is produced by hydroelectric and nuclear. There are essentially no diseconomic factors.