But your choice to avoid Alistair being the one to performed the ritual with her would be respected. Alistair was not the father of the OGB. Someone died. But how strong or determined of a character do you think Morrigan is that she would pout a bit and then just go away? Wasn't she using you all for a reason? And what do you think that sly smile was at the end of the scene? What was the purpose for that animation to you?
I agree that Morrigan should have been strong and determined enough to complete the ritual without the warden's help if it was truly possible for her to do so, but she couldn't, as has been stated many times by the game creators. It simply wasn't possible without the help of the warden. I personally think she also should have been intelligent and shrewd enough to have presented the idea of the ritual in a manner that my warden could have accepted, but she did not. Perhaps, as you say, she was merely using the warden all along, and her claims of learning to see my warden as a sister were all lies. Or perhaps she really did develop a softer side and that caused her be unable to force the ritual on unwilling participants. In any case we are both attempting to read emotions and motivations into a scene which simply doesn't show them. You see a "sly smile" as Morrigan turns to leave. I see a blank, possibly frustrated, unreadable expression.
Are you at all familiar with psychology experiments where a picture of a person with a blank expression is presented, and test subjects are asked to interpret the emotions of the person in the picture? People almost always project their own emotional state upon a blank picture. This is what is happening here. You want to see Morrigan triumphing over the warden, so that is what you see, even though there is no smile.