Again, my comment is not about what Teryn Cousland would have done, but about Loghain believed he would have done. Loghain was paranoid. And remember neither him nor Anora are noble by birth, they are just commoners that Maric elevated. Loghain in his paranoia would have believed that the nobles would not support him.
Also, I am not saying he intended to kill Cailan, but it certainly appears that he wanted to remove anyone that Cailan trusted so that Cailan had to rely on him, thus insuring no deal would ever be struck with the Orlesians. In his paranoia, he had concluded that the Grey Wardens were involved in a secret plot with the Orlesians as they had suggested waiting for the Orlesian Grey Wardens. Loghain's paranoia drove him to do despicable things. He never had any intention of charging that field in Ostagar. He had already decided the Grey Wardens were enemies and needed to die. That Cailan was stupid enough to side with the enemy, sealed his fate, as Loghain says "Your faith in these here Grey Wardens will be your undoing." Paranoia is what caused everything he did, I am not saying whether that is a reason to forgive or not, but it is what it is.
I chose to kill him simply because I have a motto, "Try to kill me and fail, you just signed your death sentence" I never let anyone who tried to kill me, live. Hell I didn't even know that Zevran could be a companion until after my 3rd playthrough, when I saw it on the wiki.
Loghain was not given any reason not to trust Teyrn Cousland as they are both veterans that love Ferelden and did fight to protect her from foreign occupation. You're trying to make Loghain seem like a madman when he really wasn't, his reasonings for what he did at Ostagar were very logical and we do know that he has a huge distaste for assassination. With the Grey Wardens he had no choice but to employ it, at Howe's urging, to help curb a rising rebellion. While with Teryn Cousland he would have zero reason to employ it, heck having Teryn Cousland at his side would have GREATLY benefited Loghain far more than Howe killing the man as Teryn Cousland was a Hero and a Royalists thus he would serve Queen Anora and bring much favor to Loghain's cause.
His fear of the Orlaisians isn't without cause, they treat Ferelden as if it's still their's and show no respect to them. Loghain also knew about the push for Cailan to abandon Anora for a certain Orlesian. If Cailan had married the woman than it would have firmly handed Ferelden over to Orlais upon a silver platter.
You misread Loghain, he doesn't blindly hate the Grey Wardens. What he hates is that they have so much pull on the King when there hasn't been a Blight for many years and Loghain believes in reality over long dead Legends. As far as Loghain was concerned they had outlived their importance and had no right to dabble in Ferelden politics hence why he sends orders that the Grey Wardens leave Ferelden or suffer the consequences. He doesn't organize manhunts for them and he does give them time to leave which they do.
This is due in part because the Grey Wardens were now in backing with the Orlaisian forces thus if Loghain let them into Ferelden than the Grey Wardens would, by their rights, bring the Orliasian forces as well since they had committed to the Grey Warden cause.
How many of your companions did you kill? NPCs? I wonder, since a lot of character try to kill you - including companions.