1) it is the official patch from SI, updated transfers, bug fixes etc etc....
2) yes you can if you bought the game Cd/Dvd or via Steam? if you are using the cracked version (which is illegal and extremely frowned upon here) you cannot
3) it will update all the bugs etc and the save game (but it will not of course load all the latest transfers etc) to get the full update you would need to start a new game
oh and you are welcome....