On the Vivienne subject:
Let us not forget the Rivaini Seers! They are not alone in representing a harmonious coexistence between magic-users and non-mages in Thedas, but they are definitely a good model which proves that freedom in magical practice is not doomed to follow the footsteps of Tevinter. As I mentioned earlier, I think there is something to be said for the fact that Tevinter actually uses the same Chantry model, if in a twisted form, they cannot escape the axis if they only gravitate from one polarity to the other, mirroring the other Chantry almost to perfection. The male Divine, the templars who are more like gang enforcers than a policing force, circles that work in the reverse, etc.
I also had a revelation regarding Tevinter mages: I think it also significant that a lot of their magic centered around the worship of the Old Gods. Historically speaking, this could be the point where magic and the use of magic began to be focused on what we consider corrupt and dangerous paths, facets of which remain alive today despite their "conversion" back to the worship of the Maker. Perhaps this can be attributed to why Tevinter is exemplary of the kind of magic that is an evil that must be contained, according to Chantry beliefs.
All in all, I think Vivienne is really just not in a position to truly judge whether or not magic would be a destructive force if allowed to run free, so to speak, because she is incapable of thinking outside of the context she was raised in and, as pointed out, her personal experience. As Solas says, the Fade adapts to the mind of whoever comes into contact with it, i.e. if Vivienne was taught during her entire life that demons lurk in the Fade waiting to consume unwary children (and even were she not a mage, in a land ruled by the Chantry, every single child is raised according to this belief), then it is inevitable that her encounters with the Fade would result in potential disaster/possession.
As to her idea that lynch mobs and the like are the inexorable result of letting mages run free, I have no doubt in my mind that such things would occur, at least in the beginning. It's been centuries since the last time the people of Ferelden, Orlais and the Free Marches have had to be in direct contact with magic wielders, so naturally both sides would be incredibly inept at coexisting. The more dangerous element of this volatile mix is the fact that mages are deliberately kept at an immature level - they have become incapable of defending themselves on many levels, despite the power they wield, which is dangerous since the moment the common folk turn on them, they have little other choice as they see it than to lash out with the power they possess, not to mention that they still have very unfortunate belief systems regarding the Fade and magic in general, which means abominations will run rampant. However, I do have some faith in them - if they stick together and take preventive measures, perhaps communicate with those mages who are free (e.g. Grey Warden mages), then they have a chance to start learning about both themselves and the world. Vivienne cannot see this, however, and she has her reasons for not believing.
All in all, I find it no accident that a former Grey Warden pushed for mage freedom and won in a sense, nor do I find it an accident that they landed in the fire after clambering out of the kettle - yet the Inquisition has the ability to represent a new accord with mages that Thedas has not really seen many strong examples of before, they have a chance to remake people's opinions of them and to revise what they thought they knew about the world, too. Plus, by their participation in closing the Breach, they have truly become citizens of the rest of the world and I think that at least a portion of the common folk will remember that. I saw signs of this right at the start in Cassandra's comments: not only her reaction to the mage pestering her, which accepted their new role as equals (if that is the choice you made, of course), but also in what she said about them being equals now, she chastises the mage and tells him to act like it. It is obviously different and surprisingly uncomfortable for the mages, yet they do get the acknowledgment they sought - the burdens of freedom, as it were. That brief conversation symbolized this transformation for me in many ways.