No one's claiming she isn't manipulative - she's an excellent politician. You just told her that you would not support her and you'll depose her - in which case at best she'll become a commoner, effectively, at worst, the new regent could have her killed. No one in their right mind would continue to support the Warden after that - her actions at the Landsmeet in this case are entirely logical.
It's funny that the Warden can do the exact same thing to her (promise to help her and then crown Alistair) but no one is calling themselves a backstabber here, are they?
All the Warden has shown her by that point is that they're willing to put someone inexperienced (and easily manipulated by Arl Eamon) on the throne and are going to kill her father. You're shown her your priority at that point is taking over the throne, not dealing with the Darkspawn.