Still not seeing any proof of mind control. She's incompetent, and her desperation makes her more so.
Frankly, the Mages deserved a better leader. One that wouldn't force them into a war they couldn't win and then sell them into slavery when her decisions lead them to the inevitable dead end.
Which would indicate that Fiona is not a "Mary Sue".
A "Mary Sue" would just be able to wave their hands and everything would have been better. Since that didn't happen until the Inquisitor shows up it detracts even more from that silly belief.
Her appearance at Haven doesn't really change this calculus. You claim that she has to have been mind-controlled in order to attack the Inquisition, but why is that the case? She's working for the Venatori, and the Inquisition is the enemy of the Venatori. It's perfectly possible to explain her presence without resorting to mind control. But even if she were being mind-controlled at Haven, that doesn't mean she was under the same compulsion at Redcliffe. You're making an argument from essentially nonexistent evidence.
The official guide states this for the boss fight with Fiona.
As the leader of the rebel mages, and brainwashed by the Venatori, Fiona is a formidable threat...
I believe that's where the whole "MIND CONTROL" thing comes from.
Now, I don't hate Fiona. I don't think that the label of "Mary Sue" is useful. I have no particular interest in tearing her down. I simply believe that, based on the evidence of the books and this game, she is a poor leader who made a series of poor decisions that negatively impacted the welfare of her followers.
It never has been.
People just toss it out because they dislike her character. Fiona, and by extent Alistair, are the worst examples of "Mary Sue's" I have ever seen.
As I mentioned before, if she were truly a "Mary Sue" she would have just waved her hand at Alexius when she finally realized (which she would have known at the start if she was indeed a "Mary Sue") that Alexius was going to screw her and the mages over. Alexius would then go "You're right, I was being evil and I release you from your servitude because you're so impressive" which of course doesn't happen until the Inquisitor shows up...because...the main character is generally the "Mary Sue" by default...
Keep in mind, I know you're not one of the people saying that Fiona is a "Mary Sue".