So Cassandra's supposition is just that: supposition. And if you want to only cure one person as an actual test and then watch, fine, but that's not what Vivienne did.
Assuming that's what you believe Vivienne did (or didn't) do, then perhaps Montsimmard was more judicious in it's use of the Rite of Tranquility.
Yeah but on the other hand she keeps saying the rebel mages are morons who are just going to get themselves and other mages killed.
If she showed some respect before for the mage rebellion , it would be believable she'd think tranquils would be safe with them.
But she thinks the whole thing is doomed.
Anyway Vivienne and what she would and wouldn't be able to do during the rebellion is ...an headache.
The game says she is supposed to be powerful , but everything points out at her being powerless during the whole thing.
Her leader of the Loyalist mages is a smokescreen , we never see her do anything with that position .
And mages who didn't want to get involved in the war still mostly followed Fiona .
They were morons who were getting themselves and other mages killed. But is it really a stretch to think the mages would at least try to protect their most helpless?
Vivienne was a First Enchanter in an Orlesian Circle, and the rebels were in Ferelden, ostensibly under the protection of Ferelden's monarch, and a Conclave was being called by the Divine (whom she respected) to try and bring the fighting to an end.
What's she supposed to do as leader of the Loyalist mages? Lay siege to Redcliffe herself? She kept the Templars from storming her Circle, kept the rebels from murdering her mages for not joining them, and kept the looters at bay. Not much else you can reasonably expect.
The mages who didn't want to get involved found themselves on the run or hiding in caves from mages and Templars alike.