The current standard for metagaming is woefully vague and unfortunately I don't see a good way to change that. I personally define metagaming in a manner that's more or less as vague as the definition now, but that redirects the focus of the rule in a manner which I believe is more in line with the intention behind it.
I define metagaming as allowing outside events to have UNDUE influence on a game. The word "undue" creates a value judgment that still leaves the rule itself unfortunately open to subjective interpretation; however, by focusing on the undue nature of the influence, instead of the mere existence of the influence (as the current standard stipulates), I believe it is more in line with what the rule attempts to do: prevent irrelevant matters from having relevance in a game.
Let's say, for example, that I am England in a non-anonymous game. My neighbor, France, is played by a person who has completed 15 games on the site, and been France in all 15 games, and has attacked England right off the bat in every single one of those games. I would find it hard to say that taking the knowledge that my neighbor has attacked the country I am playing in every single game he has played on the site into mind when planning a strategy is 'undue influence.' The player has an obvious tell that is made extremely visible by the player's record on the site; why wouldn't I pay attention to it?
By site definition, this is metagaming, as it takes outside-game information -- the fact that the player has, in every single previous game, attacked the country I am playing now is in fact outside of the game I am playing -- and applies them to the current game. And yet it passes my own personal standard. And I further suspect that the moderators would not punish me for admitting to taking that kind of information into consideration.
In contrast, the flagrant example of metagaming -- cross-game alliances or wars, as the case may be -- is undue. That someone attacked you in a previous game has no bearing on the current game; the actions of an agent in one game carry context specific to the game which is not replicated in the other game. If I, as England again, attack someone as France, I am not doing it without consideration for Germany, Russia, Italy, Austria, and Turkey's actions and communications within that game. That context cannot just be ignored. Thus, using that action as pretense for that same someone to attack me in a subsequent game is allowing undue influence of the events of the first game on the events of the subsequent game, and would not pass my or the site's standards for metagaming.
With that in mind...
I don't find this to be metagaming as defined to be 'undue' influence. (1) is just not true, the aim of Diplomacy isn't "kill someone," but relevant to the metagaming conversation this means nothing. (2) was a correct assessment of the board (as I would know, having been in that game). (3) doesn't definitively say anything, in fact, given the use of "I believe," but assuming it is true, it's still not unacceptable metagaming under my definition. That can be verified by checking records; as it turns out u and R are pretty good players, and having verified past experiences with available records, all this says is that the player in question did not want to attack a player perceived as strong on his own. And, as I know the diplomatic situation of that game, I know that the player would have had reason to think he was going it on his own were he to attack u or R. (That turned out to be false, but the only person who would have known that in time was me. ;) ) (4) is a logical continuation from (3). One oughtn't leave his back turned to anyone in Diplomacy, let alone players perceived to be very skilled at the game. And (5) is no bigger a tell than the England/France example I gave in defining acceptable metagaming. Just as I would factor in knowing from the past that some player always attacks country X when playing country Y, I would factor in knowing that someone is reasonable or unreasonable in my decision-making. In fact the only problem I have with (5) is that it's silly. You don't need past experience to determine if someone is reasonable. Just look at what they're saying in-game. But it's not unacceptable levels of metagaming.