1. That's what happens when you act in desperation. You don't think at 100% clarity.
Then that makes her a poor leader. QED.
2. The fight at Haven has nothing to do with anything. She's brainwashed when she appears there.
There is no evidence of Fiona being brainwashed.
4. Those were Venatori disguised as rebel mages. Even if there were non-Venatori partaking in the act, that wouldn't condemn Fiona unless she explicitly told them to go do it. The mages who follow her do it their own volition. They're free to leave whenever they please and can do what they want. Fiona has no power over them besides rank (which is meaningless since the Circle's were abolished). The mages aren't a monolithic group with a hierarchy or a hive mind.
Yes, it does condemn her. She's the leader. She is responsible for what her followers do or don't do. If the Tranquil were under her care, and while they were under her care they were slaughtered for use in a sick science/archaeology experiment, then even if she's not the one who did the slaughtering, and even if she was unaware that it was happening, and even if she would have been personally opposed to it happening and horrified at the result, then
she is still responsible because as the leader she is responsible for everything that happens in her organization. It is her job to keep her followers in line. If she is not responsible for what her followers do, then she is a
bad leader. Which is the point.
And faffing about by claiming that she didn't have any authority over the rebellion is stupid. Every single mage you talk to in Redcliffe acknowledges her as their leader. If they didn't, they wouldn't have let her sign them away into Tevinter slavery. If she's got the right and the support to enslave her mage followers to a power-mad foreign cultist, she's got the right and the support to prevent people from slaughtering Tranquil.
Personally, I think that the massacre happened
after she signed herself and her followers over to Alexius, and that the power she gave Alexius in the "agreement" was what he used to have the Tranquil rounded up, experimented upon, and murdered. He could go over her head, and didn't need to rely on conspirators and spies. Which, again, does not speak to her competence. If the deal with the Vints was supposed to keep her followers safe, then it comprehensively, catastrophically failed
almost immediately because the supposed protectors went to work killing Tranquil. She may have kept her mages safe for the moment (from a threat that I suspect was negligible, but let that go)...by trading the lives of the Tranquil under her care for them. This was probably not an explicit trade. I don't think Fiona was a monster, just horribly incompetent.