^This. Except for one thing. If the marrage with Cailen and Celene did happen all of Loghains fear of invasion would of ended.
I'm afraid I have to disagree. 
Loghain would have seen the Orlesian Royal Forces coming with the Wardens as an occupying force, even if Cailan and Anora had just wed. There was public scandal when Eamon married Isolde because she was an Orlesian. It would be much worse if their king married an Empress and began to see Orlais as his country too. As Elastion noted, Ferelden is essentially a noble's republic. Power is decidedly not centralized. While kings have always been of the Theirin bloodline, the Bannorn can and will overthrow anyone they don't like. Orlesians are by far the most hated people. Orlesian troops in their country will always be seen as a huge threat. And again, Celene had plans to take over Ferelden. Admittedly through gentler means than her predecessors or even Gaspard.
Orlais is a centralized, absolute monarchy in the style of Louis XVI France, complete with the heavy influence of the Chantry/Church, and more abused than usual peasant population.
Generally speaking in a union, the more powerful kingdom gets the most benefit. The Kingdom of Ferelden is not in any danger from Nevarra or Tevinter. Orlais is, and would request aid from their ally in those wars. Historically, their threats have been each other, their cousins the Avvar and Chasind, and most of all, the Empire of Orlais. The only benefit Ferelden gets is protection from Orlesian invasion, which it gets in return for standing side-by-side with their arch-rivals. The Occupation is in living memory, and is a collective scar for Fereldans. I do not see any circumstance in which Ferelden would stand for a union with Orlais, however hard Cailan and the same type of collaborators present in the previous invasion pushed. It would be a civil war in which Orlais would win, for a time.