Well I'm not sure about the different swear words becomming offensive at least not without research but yeah the whole 2 fingers thing I can actually explain how it supposedly came about.

Archers need the two fingers that are used as a vulgarity to shoot arrows, rather than kill strong men who could be used as manual labourers the French decided in the hundred years war to use a different tactic they simply removed the two fingers from the hand (I think it was the right hand but I can't remember) thus negating the English archers ability to draw a bow and fire it effectively. The English archers would often when they saw the French would raise their two fingers used to draw the arrows as a "haha look we've still got them and can shoot at you" taunt.

At least this is the supposed origins of that gesture.