1. 1.5 mags would be the minimum - since it is winter that may be the norm but under room temp (and allowing the mag 5-10 min to warm up) it is possible to get 2 full mags out of 1 charge.
2. yes, pressure in the mags keeps internal seals in shape
3. true japanese mags have two way fill valves (as opposed to silent fill valves). this means that as you are filling, gas will spill out of the valve. when the mag is full the liquid (visually identifiable by a thicker cloud of gas and the feel of silicon oil on the hand) will squirt out. further filling does nothing except waste propane, since the mag is filled with it already.
edit - i should mention that as all mechanical devices under stress, the residual gas in the mag will eventually leak out (6months -1 year). the best maintanence i would offer is ocassional use and cleaning