Alexius only holds Redcliffe in the alternate world scenario where Corypheus wins. As I've already said, no one could have known about the forces Alexius had behind him.
So your alternative to Fiona's decision to go to Tevinter is commit suicide, which is what going to the templars means for any mage who participated in the rebellion. I'd pick life on the run over that, which amounts to the same thing for most of them.
Right, a Magíster, given acess to Ferelden, would just come in, take the mages and leave without causing any trouble. Fiona isn't stupid enough to believe that.
She had to have known what inviting Tevinter into Ferelden would entail.
Also, Alexius holds Redcliff in all games. That is precisely what he doing at the beginning of DAI. Holding Redcliff. He took it, now he's holding it.
You are being disingenuous on two fronts.
First of all, I presented another option which would simply be "Let the Crown do its jobs and protect the realm if the Templars were really about to attack, which they weren't."
Second, no one said "surrendering to the Templars", I said "join Vivienne" which means to remain loyal to the Chantry to whom the Templars were not even beholden to at that point.
Vivienne's mages were not executed. Do you deny that?
However, there wouldn't even be any issue with surrendering to the Templars because, unlike Tevinter, they are not butchers.
I just received a War Table mission where Templars surrounded a group of mages and offered them the chance to surrender. A few tried to and the other mages nearly killed them.