1. I think the majority of the people came to the party because they were curious about the Cullen family and their house. Plus, the town of Forks seems like a really small town so I'm sure a lot of the people had nothing better to do.
2. I was not surprised Jacob came to the party. After all, he said he was going to fight for Bella. Making his prescence known as much as possible is one way of telling Edward that he's not going anywhere.
3. I think the bracelet was a little bit of both. But I think it was a good idea on Jacob's part because it was a constant physical reminder to Bella. Even when Jacob is not around Bella, she can always look down at the bracelet and think of him.
4. and 5. I was suprised that the vampires and the werewolves were able to KEEP the alliance that they created. I was convinced that it would be a good idea to them in theory, but when it actually came down to it, to actually start fighting, that they would never be able to pull it off successfully.