The game hints that Loghain conspired with both Howe and Uldred prior to the Ostagar battle as he manoeuvred to strengthen his position once the King was killed in battle. The evidence for this wasn't strong, but there was enough of it present that the theory that Loghain was planning his betrayal long before the moment of the battle was not at all unreasonable.
It frustrates me that David Gaider decided to rubbish this idea rather than be subtle about it and allow the player to decide whether or not this was the case. It's also important to note that some of Gaider's comments on the forum have contradicted lore from within the game, so they should often be taken with a pinch of salt.
Agreed so much, particularly the second paragraph. Plus, according to the Dragon Age Wiki, Mary Kirby wrote the majority of the Landsmeet and Loghain's character. While Gaider was the lead writer of the game, Mary Kirby wrote the majority of the character and confrontation of said character we see in-game. What she envisioned while writing and what Gaider envisioned after the game was completed could be two different things. Considering Gaider wrote his book after the game was out and much of it contradicted what we saw in-game (*cough*Alistair being born while Maric was married to Rowan in-game but being born a few years after her death in the books*cough*), and considering Gaider starts statements by saying "This is how I interpret Loghain's character/actions" (e.g. admitting he's not the ultimate authority on what happened), I tend to take it with a pinch of salt too.
Plus, to be honest, a lot of what Gaider says doesn't make sense when you look at evidence in-game. I don't think it makes sense that Howe, Uldred, and Loghain all decided to usurp the power of someone they normally have to answer to (Howe to the Couslands, Uldred to the Circle Templars, and Loghain to Cailan) all at once, all without prior consulting, planning, or coordinating. Individually, every endeavor would cause a major backdraft from the allies of those they killed. The nobles would call Howe out as a traitor and see through his paper-thin excuse of "but they were traitors" and Loghain out as a usurper that deserted under the weak pretext of "it was a tactical retreat" and "the Wardens led the king to his death!", and the Templars would come down on Uldred like a ton of bricks. We're supposed to believe they did these things independently, spur of the moment, somehow expecting they could just get away with causing the deaths of people powerful and influential WITHOUT getting called out on it, WITHOUT needing prior assurance that some powerful ally (or allies) could shield and support them?
In Ostagar alone there are many hints that Loghain (and possibly Uldred) planned the retreat beforehand. After Ostagar, there are again many hints that Howe and Uldred slaughtered who they did because they expected Loghain to support them afterwards. And that Loghain, in turn, had already promised to pardon and promote them in return for their support of his acquired regency.
I can understand the developers keeping it vague so the players can draw their own conclusions. I can understand still presenting Loghain as a sympathetic, tragic, fallen hero that genuinely believed what he was doing was right, but was blinded by his tragic flaws (pride and paranoia), and possibly also goaded on by poisonous friends. (HOWE!) I think it's possible for Loghain to leave Cailan to die and remain sympathetic since, from Loghain's perspective, Cailan WAS planning to leave his daughter to marry the Empress of Orlais, was going to let scores of Orlaisian Chevaliers into the country while they were vulnerable with the darkspawn invasion, and Cailan was already letting in "Orlaisian" Grey Wardens. I can understand Loghain fearing for the country's independence from Orlais and deluding himself into thinking he and the nation alone could defeat the darkspawn without any "Orlesian" or Grey Warden help because of his pride and self-delusion.
I just feel like Gaider went overboard trying to make Loghain sympathetic. Rather than just letting Loghain be a tragic fallen hero that made terrible choices, even if he truly thought they were right at the time, Gaider still felt the need to go: "No, wait, Loghain never did those things at all. He never planned to desert Cailan beforehand. That was just a spur-of-the-moment decision" and "Loghain never helped Howe with the Cousland slaughter. He just heard about it afterwards and turned a blind eye because Howe was useful to him." Yeah, because who doesn't trust a traitorous social climber that betrayed, slaughtered, and usurped the last people he was chummy with? Who did so without thinking about the political ramifications because, by Gaider's very "Loghain didn't plan to desert beforehand" assertion, Howe had no way of knowing the king and his closest followers would go belly-up at Ostagar, or that someone filling the power vacuum would shield him from the fallout of his slaughter.