Now I'm sad.
As for god mode, that code _broke_ with the new bukkit update. It set your hearts to 13 instead of 10, and let damage from mobs go through and kill you while other sources of damage reset your hearts to 13.
Now, death from flyhackers and whatnot is from moderators using /tpo to teleport to them instead of the expected commands (/c and /show <username> to figure out which one) to do so, which provide you with the same type of temporary invulnerability you get when you log in.
The grief system currently spawns a _ton_ of stuff. It also uses items on the ground nearby, items in the inventory, and items in the safety chest in order to repair greifed stuff with.
If a creeper goes boom near your stuff while you're online, and you're within render range of it, and you are not afk, the system assumes that you are allowing creepers to help you do stuff. The solution is to kill them.