It is David's story, but if Loghain didn't decide, then why did he poison the Arl ahead of time? Was that "just in case" he wanted to betray his King? Did David say whether or not Loghain knew his underling was going to go rogue and betray his overlord, kill his wife and attempt to kill both his children? Whatever Loghain may have consciously determined, I think he really knew what he was going to do. I mean, how was he going to explain all the Shenanigans in which he was involved? Was he going to pin it all on Howe? "Gee, Cailan, I didn't know he was going to slaughter the Teyrn, really!" Loghain is a STRATEGIST! He said those words knowing it would egg Cailan on, at least in my head canon. I give him props for being good at malicious mischief, but I honestly think he planned treason one way or another. I thought he was a great MacBeth!
Loghain didn't know about Howes plans Gaider confirmed this as well. Loghain is a master strategist on the field as we're told in The Stolen Throne, but I don't think the guy has much skill when it comes to politics and intrigue, thats Howes game and thats why Loghain relys on him in such matters. People say he should have done something about Howe but by the time Loghain returned from Ostagar Howe was one of the most powerful people in the Kingdom, He controlled Amaranthine, Highever and Denerim. Tbh I feel making an Enemy of Howe, especially after a civil war with the bannorn had just begun, would have been a huge mistake.
As for the poisoning of Eamon, tbh idk this one but i'd assumed the following:
He had that elf in the tavern on standby to inform him how the Arl was, the poison wasn't actually fatal it was just to get Cailans most powerful supporter out of the way and Loghain was going to provide an antidote later. (Though tbh I wish the poison was fatal)
Loghain was planning for a showdown with Cailan and challenging him after eroding his support base, he was going to demand they withdraw from Ostagar as Loghain made it clear he didn't want to be there, the Darkspawn forces kept growing in number with each new attack and it was becoming evident they couldn't hold it. And he was going to demand Cailan close the borders to the 4 legions of chevaliers on their way.
But as I said this is just speculation, we don't know much regarding the poisoning of Eamon.