Loghain didn't order the Cousland Massacre, Howe did it with Loghain in the dark the writers also confirmed this And how exactly does that play into Loghain's plans anyway?. When Loghain returned north with the remains of the army Howe was then the most powerful man in the kingdom bar himself, with control of the capital, Highever and Amaranthine. To make an enemy of Howe at that point would be suicide.
The King participating in the battle was not part of Loghain's plans, hence why he protests in him taking part in it in the talk after your joining, his words making it clear this isn't the first such protest.
I'd suggest reading this as to the other details regarding Ostagar: http://social.biowar...308/blog/10285/
As to Eamon's poisoning, yes that did happen before the battle. We know little about it though so much would be speculation. If I were to speculate i'd say Loghain did it to erode Cailan's support/power base so as to challenge Cailan and force him to forbid the 4 legions of Orlesian Chevaliers from crossing the borders and to move back north (Loghain didn't actually want to be at Ostagar, he viewed it as little more than a darkspawn raid not a blight)
My source on the above Howe statement as well as about the other things confirmed by the writers were from the Leader writer David Gaider, wasn't in an interview it was from his posts on these forums back in 2009. Can't recall the exact threads but it may have been the Loghain support thread which would be in the archives now, Will be hard to find now since they switched to this new forum and alot of the old links dont work anymore but will have a look around for them
I am slightly torn in continuing with this, as this isn't really the thread for this discussion. Yet obviously, since I am responding, I am continuing. Partially because I find it really annoying to accuse other people twisting things in headcanon while basically offering nothing as headcanon as proof.
First, the link. It actually means nothing as it is pretty much assumptions of what was happening in the fight. Nothing in the game supports that reading of the situation over any other.
Second, you didn't provide any actual links to these dev statements, which I understand are probably difficult to find, but you can't really expect anyone to take that as evidence in this case. Especially if we are talking about something that Gaider said, if nothing else than for the frequency his statements are misquoted. Thus we need to look at what happens in the actual game.
Third, so wait, Loghain went through with a plan to poison one of the highest nobles in the land, but wasn't treasonous towards his king? And somehow doesn't make it seem like he would be okay with his political enemies being removed by questionable methods?
Fourth, the only way Howe's plan makes any sense is if he knew of Loghain's plans to betray the king at Ostagar. Otherwise the army would have returned back and found one relatively minor renegade noble just wiped out one of the big three noble families in the realm in a brutal massacre. What exactly would have been Howe's plan in this situation if not for Loghain? How was he seeing himself getting out of things? Especially since we constantly see Howe by Loghain's side during events and Loghain actively giving Howe more power?
My main issue in this discussion isn't the fact that people like Loghain, for me is that a lot of that praise overlooks a lot of the stuff he did and we saw in the actual game, such as selling citizens as slaves, while being at times, such as here, really aggressive towards others who don't see him in the similar manner. Especially since all the supporting evidence his hypothetical situations, head canon and dev statements which are often referred, but can't be linked to.