I wonder: has anyone suspected mind control as the reason behind Fiona's decision? I mean, the subtle kind that shades your perceptions just enough to make a really stupid decision that goes against everything you fought for in the first place?
Because Fiona's alliance with Tevinter makes no sense, at least not on the terms they were offering. Fighting for freedom but accepting servitude to another, I can't see a passionate freedom fighter like Fiona making such a decision.
As far as I'm concerned, she was mentally nudged into it by Alexius. She does mention she feels strange after all....
I've heard the theory raised. I've never seen any evidence for it, and even less argued support. At this point it is little more than fan-based special pleading for something to excuse Fiona from not living up to expectations as a noble revolutionary.
In Bioware games, mind-control has always been deliberatly blatant in many respects, from symptoms to companion alerts to point it out to post-brainwashing blatant exposition about it's existence. Bioware has never done brainwashing subtly, and there's nothing to suggest it mysteriously started in just one case of DAI despite numerous other blatant brainwashing tropes (Red Templars, the templar-route enslaved magister, the note of Venetori brainwashing the Circle mages).
No one, not even Fiona (who has the greatest interest in disavowing responsibility) brings up blood magic regarding Fiona's actions. On the other hand, there is an explicit plot device regarding the confusion of Fiona's appearance in Orlais, and that was the use of time magic. Which may be an awkward plot device/hand wave, but it is explicitly present, and presented as the basis for Alexius's timing and leverage over Fiona. Blood magic is never even insinuated by anyone in the cast, protagonist or antagonist alike.