Desmond ran over Locke knowing that Locke would be taken to the hospital to meet up with Jack and therefore remember his life on the island.
There's no evidence to suggest that the bomb exploding actually created the alternate timeline. That was just an assumption made by everyone watching (me included). When we found out what the alternate timeline was, it made more sense. The bomb was just that - Juliet was next to it when it exploded, and she died, the rest survived.
Some of those in the LA timeline you've already seen die over the course of the series. For the rest, we don't know how they died, i.e. Kate, Sawyer, Miles, Frank, etc. Presumably they got home and lived normal lives. We know Hurley was the island protector and Ben was his lieutenant, if you will, but we haven't seen any of that either. Although there will be an extra on the DVD, apparently, which shows some of Hurley's time as the island's new guardian.