I haven't taken Loghain through RtO yet, but I hope to do so tonight or tomorrow. I'm eager to see his reaction to it. I ran it before I got Wynne from the Tower, so I haven't heard her reactions, either.
I have to say, the content of the Cailan's documents surprised me. I love how BioWare ordered them: 1 - a polite missive stating that military forces from Orlais are available to help; 2 - a concerned letter from Eamon that suggests that Cailan needs to think seriously about having an heir, and if Anora can't provide it, maybe it's time to find someone else; and 3 - a note written in a familiar tone from the Empress to Cailan that suggests a fair amount. Nice build-up to the reveal of Cailan's apparent relationship with the Empress.
I found Alistair's reaction to the letters intriguing: "It looks like we should have trusted the Orlesian forces more than Cailan's father-in-law." Trust them to help with the Blight? Yes, I can see that. But I don't for a minute think that Celene (and oh God, is that a reference to Celene Dion?

BioWare = Canadian, Celene Dion = Canadian...) was dallying with Cailan because of love. By all evidence, she's a political mastermind. She obviously has a respect for Ferelden that her predecessors didn't (as detailed in the Codex entry she supposedly wrote about the country), but that doesn't mean she doesn't have the desire to possess it. She's just being a hell of a lot smarter about it. The previous emperors tried to subjugate Ferelden for nearly 100 years, and it was a constant struggle and ultimate failure. However, she had the opportunity presented to her to conquer Ferelden peacefully, through marriage to its idealistic, naive King. She would be able to succeed at the task her father and possibly grandfather had failed at doing.
As other posters have said, there is no way in hell that Cailan and Celene would be equals. As Eamon points out, Anora had Cailan wrapped around her finger, and Anora is nowhere near the political genius Celene is purported to be. Cailan was interested only in being the hero to his own epic story, and had severe delusions of grandeur about his ability to be a good king. Perhaps Celene would allow Cailan to think they were equals, but he was far too naive to see the manipulation she would no doubt be doing behind the scenes.
Based on all of this, I now completely understand why Loghain did what he did. Here was Cailan, the son of his best friend and the woman he really loved, about to hand Ferelden back to the Orlesians without so much as a sword drawn. That, in his mind, would be the ultimate betrayal. The hurt and shame it would cause Anora was a secondary consideration, I think, though I imagine it was a slight Loghain wouldn't forget (and I'll know more about this when I bring Logain to Ostagar, I expect). And it wouldn't even be something they could fight this time, since the merger would be legal. There would be no invasion to push back, no forces to throw out of Ferelden. If Cailan were to go ahead and do this, there would be nothing Ferelden's nobility could do to undo it, short of desposing the King/Emperor/Empress. That would lead to civil war, but with the Orlesians already within Ferelden's borders, there would be no getting them out. Cailan marrying Celene would destroy Ferelden and strengthen Orlais.
Loghain saw this. He and Maric had already agreed that Ferelden was more important than a single man, even if he was the King (in The Stolen Throne). So, Loghain planned for Cailan's death, all the while hoping it would not come to be. However, I think that if Cailan had somehow survived Ostagar, he would have met his death on the way back to Denerim. What Cailan had planned was not something Loghain could allow to pass.