1. Depends on your system and the style of beer, I typically use appx. 5lbs which works with my system. Factors are: Length of beer line, if you are regulating each keg, if you have different styles in each keg, etc.
2. Don't quite understand the question. If you are naturally carbing, leave it alone until it's done then I purge the pressure before connecting to CO2 then bring it up to serving pressure.
3. Yes it should last a year or more, Mine usually get drunk up within a couple months at most.
Hope this helps.