You both make excellent points.
In the current game, my character did successfully "harden" Alistair, but only temporarily.
He would be no match for Anora in his current "gentle" state but I'm thinking maybe a Petrify spell...??
I have considered the expansion and I think betraying Anora and leaving her to rot in a tower creates a potentially ugly scenario for the future, since she is popular and very resourceful. The whole thing she set up at the Howe estate was a masterwork of subterfuge and manipulation, so I think she would be a dangerous opponent and a vector for possible rebellion... or worse.
Beyond that, though, to pursue this line of action would be a significant violation of my character's values. The only real options I see is to support Anora ruling alone or support Alistair and Anora getting married. If we return to the injunction that the main task of the Grey Wardens is to stop the blight, and to do that the Landsmeet must be won and Ferelden must be unified, these are the only two options that logically make sense to me. I have read many, many posts denouncing Anora, but these are typically assessments that are emotionally or personally based. What about the bigger picture? Sure, my character would like nothing better than to ride off into the sunset with her man or, baring that, perhaps be queen. But that does seem very self-serving.
Granted, my character doesn't care much for Anora either, but the role playing challenge is to consider what will best serve the goal of ending the Blight. I cannot see that happening with Anora marginalized and the situation polarized.
I am aware that Anora will not marry Alistair if he kills Loghain, which is why my character will handle that task - happily.