also, yeah, re: those spoilers: Loghaine was actually right. IIRC it's briefly confirmed in one of the DLC packs, which i played none of because zzzzz.
His nefarious actions after Ostagar and before the Landsmeet really don't make a ton of sense, though.
"We're pretty far along as it is, man"
This is revealed in Return to Ostagar.
Basically, here's the deal with what was going on in the Loghain/Cailan equation. Cailan marries Loghain's daughter. Cailan is a spoiled brat who does not have the chops to run the country, but Anora is a master politician, so she runs the country while Cailan is screwing around playing pretend. The bannorn all know this is going on. Well, it turns out that around a year or two prior to the start of Origins, Eamon (and possibly other members of the Bannorn) were pressuring Cailan to put Anora aside and take another wife - namely, the empress of Orlais. And Cailan had been having 'diplomatic' meetings with the empress, which were becoming increasingly more familiar. During the human noble opening, there is a delegate from Orlais staying at your parent's house. It seems to me that the Orlesians were probably trying to manuver themselves politically into controlling Ferelden, through the Bannorn, as the Bannorn are the primary ruling force in the country. Loghain saw through this and was trying to put a stop to it. Hence, poisoning Eamon and massacreing the Couselands.
As for what happens during the landsmeet, Loghain had no intention of killing Anora. Anora used it for political manuvering because she saw the writing on the wall. She knew Eamon was moving to dethrone her father and had a good chance of success, and her only way to ensure that she retained her power was to cut a deal with the Grey Warden. Knowing the Warden wasn't going to break her out simply on the grounds that her father was keeping her locked up while his known enemies were in town, she BS'd the story about her father being willing to kill her to gain sympathy. (It worked.)
