Here are my thoughts on rules for the player interactions:
1) Respect the stuff other players made. You may steal their artifacts and kill their dwarves and adventurers, but do not do anything that completely ruins what they created on purpose. If you take over a fortress, you are not allowed to flood it or do anything that makes it impossible for other players to conquer it back. You can hide artifacts in your own fortresses, but do not wall them in or destroy them in magma. It is part of the world and it is not to be deleted from it without its creators permission on purpose.
Do not hide stuff at the bottom of the ocean.
2) Act in your factions interest, your are a team after all. If you plan to betray them and join the enemy you should reflect that in your storytelling.
3) As for the storytelling, write down everything. If you want to keep something secret, like the place your adventurer is currently hiding, you should atleast give some hints for the other players. I don't think anyone wants to waste his whole turn looking for the hole you hid yourself in so they can take revenge for you eating their cheese.
4) Try to stay 'realistic'. If the enemy has built a fortress at the passage to another continent so your civilisation 'can't pass without their permission' you shouldn't just build your own fortress on that continent. If somebody built a huge bridge over the ocean and wants you to pay taxes for using it, do not just swim past it in adventure mode, even if you can. You either pay the appropriate price, try to sneak by or pick up a fight.
5) Remember there is non-lethal combat. Think twice when you are about to kill the other player's favorite adventurer.