So if someone kills the Ender dragon or Wither they get 5 points. Why not first to do it gets the points
I'd go with number 2!
Still, this seems like a bad idea, how do you know they aren't in creative mode spawning in stacks and stacks of diamonds and building a base so it looks legit? Only way around is for them to record (not everyone can), stream (not everyone can), or have everyone on the same server (diamond thieves). No matter what you do, nothing works!
It's quite easy to look in the logs and hand the logs out to the players after the match is over, to prove to them that it was fair game.
Also, the point is to be competitive, so you can steal diamonds from other people as well as other more important things.
I would like to see a way somebody can spawn in items without being opped and without being on a creative mode server like VoxelBox. I doubt there even is a way, since server has to approve of every item that's spawned in. And it checks the ops.txt file to get that information about who's allowed to spawn things in and who isn't.
Again, it's very easily recorded if somebody does spawn something in somehow because the tournament host gave them op status to give them an unfair advantage.