You're right that we don't know Calain and Loghain knew the Wardens' plan to use the treaties, much the same way my wife wouldn't know I used a pen at some point during inventories. But one can logically conclude that both examples are extremely probable.
No it's not. Duncan would not tell anyone that the treaties exist until he finds them because what would be the point? "Yeah, there were some treaties the Grey Wardens made that we could have called upon...if the order hadn't lost them...just so you know, I blame the fact we were exiled for two hundred years." Once the treaties were found, Duncan proceded straight to the Joining and you weren't passed out for long or Alistair and Duncan probably wouldn't have still been staring at you. After that, you and Duncan go straight to the meeting. Duncan didn't make it there very long before you and if he had informed them of the treaties they would probably be talking about that instead of bickering about Cailan remembering or not remembering the battle plan.
Also, if Loghain knew about the treaties then why wouldn't he have people on the lookout for the Wardens at Orzammar's gates and the Circle Tower? If he was worried about the Wardens it only makes sense to worry about the army he knows they can call upon.
The treaties were given a tremendous amount of weight, which is shown by everyone's reaction to them. That there is a problem to solve before hand doesn't detract from this fact. It's simply built into the game so you have quests. And being that Loghain is a longtime, afluent Fereldan and an astute politician, you really think he doesn't know this? Come on.
Loghain is not and never was an astute politician as the game well shows.
Also, those treaties are four centuries old. The people you talk to barely remember them (Vartag's 'I vaugely remember seeing those in the Shaperate', for example) and though they all essentially agree to support you, I don't see why it's stupid to not automatically assume that four centuries old treaties for a Blight you don't believe in will be immediately and unquestionably honored (which they're not. Really, the treaties just get you access to the treaty groups and they'd follow you without the treaties by the time you're done).
I was referring to Eamon. And why do you have to add, "which given the demon's intervention, we don't know if it would have killed him"? What difference does that make? Why sugar coat it? Loghain. Tried. To. Murder. Him. And I guess we'll have to agree to disagree about whether or not Teagan would unite his troops under a man who murdered his brother.
I'm not sugar-coating it. At all. Word of God says that Eamon wasn't supposed to die. You may or may not accept WoG but I do so I'm content to believe what they say. Also, they sent an elf (whose name escapes me for the moment) to keep watch and report if Eamon was dying so Loghain could send an antidote...which admittedly he may have not done if the civil war was still brewing.
The difference it makes is that while poisoning someone and leaving them in a coma is still a horrible thing to do, it is better than attempting to poison him to death. Still no sugar-coating.
And don't 'I guess we'll have to agree to disagree about whether or not Teagan would unite his troops under a man who murdered his brother' me. Why the hell would EAMON unite his troops under a man who poisoned him? Teagan doesn't have to unite his troops under Loghain and Eamon certainly doesn't. He just needs to inherit his brother's troops.
As for Loghain, we'll never know the possible outcome had he not betrayed the king. But even prior to the betrayal, I don't think anyone would have had any doubt that the death of the king and all the men at Ostagar would have resulted from his actions. He deserted knowing the king would be killed as a result. And YES, sacrificing his men to save the king would have been the wise decision. Better to have a civil war, or a single unifying figure?
If the king would not listen to him telling him to stay off the front lines where he would be in some of the worst danger, what was Loghain supposed to have done? Drugged his food and kept him off the battlefield by force? Loghain, not being particularly good at politics, did not anticipate a civil war and if he hadn't been so heavy-handed in demanding the Bannorn accept him as regent (or if he hadn't tried to be regent and just let Anora rule) then it's entirely possible that it could have been avoided. A king dying does not always lead to a civil war. There wasn't one when Maric died, after all.