It is Fiona alone who decides to ally with Tevinter whereas the majority of mages you talk to don't agree. So yes, she betrayed their trust.
Then, I don't think neither side is evil nor good, you can choose who you prefer.
If Leliana becomes Divine (ending spoiler) :
1)Fiona herself (unless you consider her lying) speaks of there being multiple supporters, that Tevinter spread whispers around the rebel mages quickly. She decides, but she does it with support. If she had no support then I'd totally admit that, but what I've seen says otherwise. It may be primarily her fault, but one can understand her situation enough to get what she was trying to do.
2)I didn't say either was good or evil. I totally agree that you choose who you prefer. However, I do think the narrative has a degree of bias. I don't think this is a bad thing. It just creates a varied interaction with the players' personal biases.
Example: Narrative in DAO goes that the Circle Tower can be saved and remaining mages spared. Thus Templar concerns are ultimately unnecessary. Yet one is allowed to be on the Templar side even as the narrative is more biased towards Mages. They're given enough reason to be, to choose that choice of Templar, but we know they don't need to do it.
In DAI its the first time I can think that its rather equal on both sides in the narrative. Fiona may have even called the Tevinters, but others could have fought against that and overruled her. But they didn't. They were tired and losing and while Tevinter wasn't their idea and it isn't something they support, as a faction they allowed it.
3)Yes I'm most for that ending outcome for the mages. I'm not so sure about other aspects of the larger outcome, but for mages I'm all for that. I may end up going with
instead though, we'll see..