To answer your 2nd and 3rd questions. yes you can install a tank and no it will not mess up the pressure switch/pump. It most likly will not solve your problem. You state that your pressure switch is set at 40-60 and that the pressure at the water heater is 20-40. It sounds like your water heater is 46 feet higher than the pressure tank and controls. It takes .43 pounds of pressure to raise water 1 foot. Your options if this is the case are to raise the pressure by 20 pounds or to add a booster pump at the house. Before increaseing the pressure you need to know a number of things.
1. pumping level of well
2. performance curve of pump
3. safe working pressure of tank
4. size of line from well to home (friction loss)
5. elevation from well/tank to house and all other uses
6. pressure relief valve setting at pressure tank
If any of these items are inadequate they would need to be addressed to decide how much pressure to add and rather that pressure would be safe. In addition increasing pressure needs to be performed along with the coorinsponding adjustment to the air pressure in the tank. A booster will work but you still need most of the above data to size it correctly.
Hope this helps