is there any way to not let the used co2 get wasted and rather restore it in the main tank?
Short answer is No, once used its exhausted.

Your second question re air instead of Co2, well you could use HPA, but you will not have the same volume of gas as you would with Co2, this is so because at around 60bar Co2 becomes liquid. Air on the other hand does not, and rules specify that pressure should not exceed 68bar.

A single acting ram will obviously consume less gas, don't know if there's any difference in price between single or double acting types, but most you'll find will be double acting.