I'm with KariTR. I always wondered if you absolutely have to do the Redcliffe quest arc all the way to dealing with the demon to get to the assassin or if there are other ways. Shorter ways. Much, much shorter ways.
As to another post here, somewhere, my girls don't have a problem choosing either. Zevran it is. All the time

My mage (first play-through) went for Alistair, who, in the beginning is very charming and appealing. She did everything she could for him, even to the point of setting asider her feelings about certain issues.
The first rift came when my mage decided to allow Isolde to atone for her crimes by sacrificing herself for her child. She did this for several reasons:
a) Isolde's actions set the whole horror in motion

Isolde acted without keeping her people in mind. She's not an average wife/mother, she's the arlessa
c) I have the habit of letting my PCs talk and listen to just about everyone, at least on first play-through. She felt so very bad for all the people in Redcliffe who lost loved ones, and who where so very terrified. she was impressed that they took up arms to fight for their village nevertheless. That more or less showed my mage the full horror of what had befallen the village.
d) Isolde lied over and over again to Teagan and to my mage in order to hide her crimes (hiding a mage, and hiring an apostate), and her son's affliction
e) Gossips and rumours where abound by this time that all was not well at the mage's tower
f) How could my mage know what the demon in Connor would wreak if not dealt with on the spot. As in now, right away.
So it made no sense to her to go to the tower. She can't explain to Alistair, and he won't listen to any other reason. She also sees now that the whole thing is not about her but about Alistair and Eamon. She's shocked, but things calm down and she forgives Alistair (again).
He gets colder the deeper the romance goes, and brushes her off after sex, when she asks about the future.
What the hey is it with this "my dear" stuff? Hated it. But my silly mage went on with the romance. Then she faces the ultimate humiliation. She gets dumped, and I'm sorry, the reasons he gives sounded bogus to me and my PC alike. To top the humiliation off, she finds that he can be 'persuaded' to have her as his little piece on the side. That's when she turned away from him.
She was flirty with Zevran. I mean he's charming and sexy, so why not. She never went further, but somehow or other she got to 100 with the elf. I forgot to look whether the romance was on, or whether it was loyalty and friendship only, but Zevran is the one who stands by her, and consoles her (so to speak). My heart-broken mage finally saw the light, and realized that she gave her love and affections to the wrong man.
From that first play-through on, Zevran was it. He always will be.
And that, in a nutshell, is the appeal of Zevran. His loyalty, his depths, his never faltering love/friendship.