1)Keep them for BG2, and collect the other pairs
2)Just one of the acts of comedy in the game, can be sold for a lot of money if you know the right place.
3)I can't remember this one, where did you get the letter from?
4)Yes, it changes your gender and therefore is cursed. Can be quite fun though.
5)My advice is don't kill him since he doesn't leave anything good, and he sells a lot of good stuff when he is alive. Also, there isn't really a quest, and you don't actually get a reward for doing this.