If you're using blood magic to put the whammy on someone and they notice what you're doing... you're kinda doing it wrong.
Both the Venatori using it on King Markus and Sophia Dryden having Avernus give the nobles a little "push" to join her rebellion, only worked because no-one saw what they were doing and thus the people affected had no reason to suspect they were acting contrary to their own free will.
In DA2, when Idunna uses it on Hawke, part of the reason they were shown to struggle against her control was because they could see what she was doing.
And yet, Dragon Age writing has consistently approached blood magic from a narrative perspective of 'let's make it really obvious to the player.' Induced insanity is consistently a point of emphasis with blatant codexes or character conversation, not left to meta-inferrance in the absence of others. Every time we deal or address magical mental influences, the game is bloody obvious- including, ironically, the very examples you cite. Dragon Age has not been a story in which 'blood magic influence' is the default rational for antagonism. When the writers want to indicate that someone important (or even someone unimportant, but part of a story plot) is under mental influence, they tell us that. Be it Blight contamination, or demonic possession, or demonic insertion, or idol-induced paranoia, or magical compulsion seals, or... yes, blood magic.
The argument of 'Fiona is a victim of blood magic' not only ignores that there is a standing, often-raised basis for her decisions (Alexius's time travel shenanigans, her own history of decisions), but it also ignores how the Dragon Age writers have depicted Blood Magic in every other case... which is to say, how absent the suggestion of blood magic is. This isn't just a matter of 'Fiona's opponents don't like her'- no one in the story suggests Fiona is under blood magic compulsion.
The Inquisitor can't. Experienced mages like Solas and Vivienne don't. Dorian, who's been spying on Alexius and is familiar with Tevinter blood magic plays, doesn't. Alexius doesn't, not in the bad future (where he for some reason doesn't brainwash her unlike the alleged present) or in the current timeline (when he's given up all hope and resistance). Alexius's son, who's within Alexius's inner circle and helps the Inquisitor and Dorian, doesn't. Circle Mages, well placed to notice sudden or arbitary changes in Fiona, don't. Leliana the spymaster, conspirator master and outspoken pro-mage, doesn't. Experienced diplomat Jospehine doesn't. There's no codex, or background dialogue, informed or otherwise, alleging it.
But, most of all, not even Fiona argues that she was ever under blood magic influence after the fact. And considering Fiona's actions to that point- betraying Ferelden and selling mages into slavery and standing by during the Tranquil genocide and offering empty protests but no actions to Alexius's actions and intentions because she's already committed- Fiona should be the first person to want to argue that it wasn't her fault.
But she doesn't.
When no one- not the alleged victim, not the would-be perpetrator, not the collaborators, not the mole, not the spy master, not the observors, not the peers, not the enemies, not the friends- when absolutely no one argues that Fiona is under mind control influence-
-when the only people who do are fan sympathizers who argue on standards of inference that dismiss every other profered explanation, that ignore the narrative tools habitually used in mind-control circumstances, that ignores a major narrative theme of how the arch-villain didn't rely on mind-control to control or influence the leaders of his pet factions-
-then, by Bioware standards, there's no narrative support to believe something as paradigm-shifting as this happened.
It's the indoctrination theory all over again.
Really, because Fiona offering an alliance to the Inquisition in Val Royeaux was actually a smart and practical move, wasn't her fault that Alexius Control-Z'd the event from happening to her, nor was it really her fault that he kept changing the deal after they'd struck it?
Corypheus already had threatened the lives of her fellow mages by killing the dissenters who refused to join the Venatori, then ordered the rest into an attack on Haven... which goes against both things that Fiona wanted to do, keep her people free from harm and free from any kind of military action.
As for all the bodies, I pointed out a couple pages ago there's the lake or the Hinterlands, which is already filled with corpses from the fighting already. Plus y'know, the Venatori have magic, so it's not like they couldn't dispose of the corpses simply by wiggling their fingers and turning them to ash?
I honestly don't understand what this back-and-forth has to do with anything.
That's not aimed at you- I just don't understand what this argument is supposed to demonstrate. Are you arguing that Fiona and the mages would have gone along with the attack because they felt they had no choice? Sure. Not sure what that's supposed to show.