And I think this is the problem with what the detractors are missing, no-one has been suggesting that she was under mind-control from the start.
It's a pretty old claim from some of the people in this thread, Sifr. Not you specifically- not that I remember- but I know Ieldra's made it in the past.
It typically comes up at the 'at what point does Fiona allegedly break character?' For people who tend to make the blood magic argument, it's when she does stupid things- most of which are well before the attack on Haven.
Merely that at some time during the interim period between the time that the Inquisitor allied with the Templars and the attack on Haven, Corypheus was desperate to launch a reprisal attack on Haven and thus ordered the Venatori to accelerate their plans at Redcliffe. Brainwashing the mages would be a quick way to raise an army that is fanatically loyal to him, who'd not freak out and quit once they saw who their master and his little pet was.
No-one saw anything weird in Redcliffe, because at that point, nothing weird was going on (save for messing with time). That's also why in the Dark Future, this incarnation of Fiona still is herself, as in this timeline with both the Mages, Templars and Wardens under heel, there was no need to raise an immediate army, meaning Alexius was free to keep her around and eventually turn her into a lyrium grow-bag.
Yes, it's a freaking shame that this would require it all to happen offscreen and is extremely lazy writing... but it would account for how we see little indication that most of the mages in Redcliffe want anything to do with the Venatori, until they all somehow end up drinking the kool-aid and become a legion of fanatical Elder One supporters who decided to come over and get a little rowdy?
Except there's no requirement they drink the cool-aid, because the objection of 'mages and Fiona would never go along with it' are... well, dependent on the assertion that the mages and Fiona would never go along with it actually being true.
'Fiona would never go along with an attack against the Inquisition because that would be strategically stupid.' 'Fiona would never make deals with Tevinter, because she was a Tevinter slave herself.' 'The mages would never accept slavery, because that's why they rebelled against the Templars in the first place.' 'The Circle mages would never tolerate blood magic and power-mad mage supremacists.'
These are all the same sort of assertions, resting on the same sort of base- the assertion of the character of those involved... and every single one of them was wrong. Fiona would make strategically stupid choices in the name of the mages, because that's been consistent both within Inquisition and before. Fiona would make deals with Tevinter slavers, because she did. The mages would accept being sold into slavery, because that's exactly what they did when Fiona cut her deal. The Circle mages do tolerate blood magic- and the massacre of dozens/hundreds of Tranquil. The Circle mages do tolerate power-mad mage supremacists... right outside their own gates. Meanwhile, the Mages within Redcliffe are either passively going along with Alexius, because of a lack of alternatives or otherwise, or are willing participants.
We could insist that the only blood magic could explain the lack of heroic character. Or- and this doesn't require copious amounts of headcanon absent from the story- we can consider that the rebel mages don't have heroic character.
Which, and this is a huge hit to the moral superiority of the pro-mage movement, consistently gets shown time and time again in the game. A great deal of the still-active mages are mage supremacists or open to them- not only those outside the gates, but those rising in favor within Redcliffe's new regime. The mages do stand by with nary a revolt when Tranquil are slaughtered, Arls ousted, and they themselves are sold into slavery. And the mages, repeatedly across Inquisition, are routinely moving down routes that someone else dictates on the grounds that they 'don't have a choice.' The mages acceptance of Ferelden hospitality, the mages' betrayal of Ferelden hospitality, the submission to Alexius, the submission to the will of the Inquisition, the submission to the preference of the next divine.
Submitting to the Venatori isn't as atypical as the 'bloodmagic' theorists would like to argue- and unfortunately, occam's razor suggests the simpler answer, not the one that requires inventing proof. Especially when the early game repeatedly points out that a great deal of the reasonable mages- ie, the ones who can claim to represent the moral moderates- jumped ship from the rebellion long ago.
On the Templar path, it's confirmed that Corypheus was planning to use blood magic to take control of Calpernia if she ever became too indepedent... so precisely why is it so far-fetched to believe that the Venatori didn't do the same thing to the rebellion?
Because there's no indication they did, despite numerous opportunities to demonstrate it, whereas every other example of 'mind control to ensure obediance' has been conveyed in explicit terms by the narrative.
Believing Fiona is blood-magic mind-controled at just one point, in defiance of numerous narrative conventions, reoccuring themes, previous history, and writing style across the series is far-fetched because it's pleading special exception without basis.
No, I was arguing that Fiona would never have gone along with the attack, not unless she was being controlled somehow.
She was trying to keep her people safe and was vocally opposed to having the rebellion serve as cannon fodder for the Venatori... so Corpyheus murdering some of them, as well as conscripting them into an army would have been reasons she personally would never have been in his army.
If Fiona wanted to keep her people safe above all else, she wouldn't have started her rebellion by priming it for a massacre. Or abandoned her primary (and best) security guarantee for an incredibly sketchy alliance that Alexius had no real means to carry through.
By the time Alexius reveals his intentions, Fiona has no power or means to not go along with him- not without dying. Which leads to...
Fiona strikes me as one of those who'd have died resisting... the only reason you'd keep someone like her alive is to mindscrew her into a slave, to send a message to the other dissenters that this would happen to them if they continued to resist.
Not to call you out too blatantly, but- you're reverting to the point of using blood magic to justify Fiona's earlier decisions, not just the immediate pre-Haven context. Fiona didn't need to be mindscrewed into a slave- she sold herself into that with a trick and the specter of defeat.
This isn't a case of Fiona being out of character. This is a strike out on your part- Fiona, as established in the core material of the Dragon Age franchise, is not the stallwart heroic revolutionary who would choose death over submission. People confuse her leadership incompetence for bravery.