TMG, we clearly disagree that your definition of cheating/metagaming is universal. I'm not trying to be a jerk here, but it is clear you are a very good player and are used to people working with you and doing things you want much of the time, so it seems like there is at least potential for when someone doesn't do what you want and/or lies that you rationalize it as them "metagaming" against you rather than that person merely deciding to go another route and/or you not providing a convincing enough argument for them to ally with you and/or your plan was actually too trusting on your part and that other person attempting to take the opportunity to go against you. Furthermore, this whole concept I'm trying to describe is not a black-and-white kind of thing, as with most things in life, there is a full spectrum of where people are with one extreme being thinking that nobody "metagames" ever and the other extreme being thinking that everybody "metagames" all the time. I've had people tell me specifically they would not ally with me because of who I am, so I know metagaming exists, but I have not experienced metagaming anywhere close to as much as you claim it exists, which makes me wonder if what you are experiencing isn't pure metagaming. For example, sometimes I will ally against a person in a game that comes off real pushy/bossy/arrogant/etc. I can imagine in order to win the percentage of games you win that you MIGHT be at a different place on the "pushy" spectrum than some others care for, so they may ally against you because of your personality rather than GR/win%. TO BE CLEAR, I'm not trying to accuse you of anything, I'm not trying to accuse you of being pushy, I'm merely trying to say there is a possible explanation out there for why these people are allying against you other than what you perceive to be metagaming/cheating.
Just because the main goal of diplomacy is to win, it does not mean we all must approach the game the same way and it does not mean there is a single "correct" way to play the game. Some will focus only on winning, some will focus mainly on drawing, some will focus on GR while others will not. When I say anything goes, I mean people are free to play how they want within a game whether they are the Good Ally that is very content to be second place and see the end of the game or if they are the Ruthless Killer that does anything and everything in every game to solo regardless of the consequences.