The Grey Wardens want to bring in reinforcements from Orlais; that;s enough reason for Loghain to have them killed.
And you assume that many of these Wardens are doing this for a good cause? Many come from Orlais and have contacts with nobility. To assume this idealistic view that Wardens give up politics and richs for the good of all is silly.
So? More likely it would've become a dual kingdom. If Calian would've married Celene, would that mean Ferelden would become subjugated to Orlesian rule again? That seems unlikely.
With Orlais as it's ruler. Do you think the people of Ferelden would want that, considering that they have been abused for a 150 years of occupation? Many would skin Cailan alive when that happens.
Killing the Wardens will only result in losing Ferelden to the Blight. So if Loghain is successfull, everything he's being trying to save will be destroyed.
No. What the Warden did was stupid and totally out of line with the GW mantra. If they were dead and Duncan was alive, the first thing he would have done is to leave Ferelden and rally the rest of the GW's to stop the Blight's spread. Ferelden was already doomed.
Loghain could've done something about Howe's treachery; he knew about Howe's actions as he tells the Human Noble at Ostagar that he is familiar with the king's promise. When he states at the Landsmeet, that "whatever Howe may have done, he should have been brought before the Senesechal", makes him a bloody hypocrite. His alliance with Howe is only convenient when Loghain can benefit from Howe's actions.
So not bringing Howe to justice, he allows to go unpunished.
You are meta-gaming at this point. You don't know if Loghain knew enough about Howe's plans, considering that he wanted to kill Anora and blame it on Eamon.
At least Eamon isn't blind to the Blight's threat.
But Eamon is hungry for power. The Blight works well for him.
If he wins, Ferelden is doomed. Just because Loghain is blind to see the true threat of the Blight, doesn't make him right to destroy all of Ferelden's chances to oppose and overcome the Darkspawn menace. His actions will bring about the downfall of everything he cherishes and all he is trying to save.
If he wins, Ferelden is united and can fight slow the threat down till GWs come and finish it once and for all.
His views will lead to the total annihilation of everything he wants to save. Though his views may be justifiable, his actions -if successfull- will doom every living soul in his "beloved Ferelden". He will brook no threat to Ferelden's safety -and its independence- from anyone. Clearly he fails to realise that the biggest threat to Ferelden is his own one track mind towards everything Orlesian. That makes Loghain the bigger threat to Ferelden,, than the Wardens and Eamon combined. Loghain simply is too pigheaded to do what is right, even if it means turning to Orlais for their contingent of Grey Wardens So maybe this isn't an issue of legal vs illegal, but one of whose actions are detrimental versus beneficial to Ferelden.
While he doesn't see the Blight as a threat, it must be pointed out that the Blight was never a real threat. Preivous Blights were much bigger in scale and time while this Blight was small. It's like watching a hyped up action movie only to have a anticlimatic ending. Even if the Blight is stopped, Orlais still remains, the Qunari still remain, all of Ferelden's enemies still remain.