If you can save $288.00 per year and your $1,000.00 pump will last more than 4 years then you are saving energy and money. If it does not last 4 years, you have got to decide how much extra you are willing to spend to save energy. Except for being able to change the operating pressure, I still think you could size a normal pump for the flow and head to get about the same energy savings. If you are really looking at energy savings alone, you may have to look back further. If saving a little energy means replacing a pump every 5 years instead of it lasting 20 years, then how much energy is used to manufacture, transport, install, and dispose of those extra pumps and motors. A big part of what you pay for the extra equipment, is used for energy. It is like the hybrid cars. If you have to replace the batteries every 35,000 miles, the money you save on gas is spent on energy mining, manufacturing, transporting, and disposing of batteries. It still seems to me that making equipment last, is the best way to save money and energy.
Putting a ½ HP pump end on that 3/4 HP motor will reduce the horse power. It would pull slightly less power with a ½ HP motor but, the 3/4 motor will run cooler and hopefully last longer, which I think is going to be your problem anyway. 13 PSI of air in a tank that runs at 35 PSI is not going to hurt anything. That larger tank is not doing you any better job than a 1 gallon size tank, it is just being used as a shock absorber. You have to have a pressure bandwidth like on at 20 and off at 40 to put any water in a tank or get any out. When a drive holds the pressure steady at 18 PSI all the time, no water comes in or out of the tank regardless of the size. Normally the water in the tank stays in the tank and gets stale and contaminated.