[Lariren Shadow]

Part of the reason why Merry was there could also be to centralize the hobbit emotion. Keep Merry and Pippin together so that way there is no random panning to The Houses of Healing, which, actually, weren't in the original theatrical release. It's hard to have a main character in a place that doesn't really exist to movie goers.

As for Aragorn(which I now have to look at to make sure its spelled right, damn Tudor history) beheading the Mouth of Sauron, he needed to die. Who else better to kill him than Aragorn? Plus, as it has been said, it's the movie, it looks good.