If you play here long enough, you will encounter people over and over again. Personally, I never know who I will ally with based simply on the names, it is always dependent upon what works best for the country I control. That being said, are there people here who I tend to 'trust' more than others. Yep. But only as far as I can throw them! ;-) The key is to make the alliance as useful for them as it is for you. Diplomacy can be used even when stabbing to ensure the stabee knows it is simply for strategic reasons, then they are less likely hold a grudge. Everyone here is human (I assume) so its natural to get miffed if someone pulls an extremely egregious stab on you. But each game is its own, your best friend from last could be your biggest threat in the new. Going in knowing who you are going to work with is just not fair to the other players. Regarding finding out who it is in an anonymous game, it depends on WHEN they figure it out. At the beginning, that is a problem. Toward the end-game and they were working together already, not so much.
Integrity is the key to both situations being discussed. With the extend question, it is very easy for someone to notice the request, hold off in hopes it will not work, then attack in expectation that the orders will not be in for that player. There are going to be some that do that. Technically, it is not cheating. But it is still not something a person with integrity would do. Eventually others will notice though. And you don't want to get a bad rep on here. For those who have allied with me in the past, you know I could keep going on and on, this is a short message for me.