That is assuming the Imperium would go ahead and sacrifice everyone. It is not the Mordor you see, people actually live there and prosper. Even mundanes, shocking I know.
This is a good point. People seem to think that Tevinter is entirely evil which is not true. Sure, they have a lot of bad points, and I do mean a lot, but even Dorian, a Tevinter citizen, believes that his homeland can be saved and is worth saving, which is why he wants to go back there near the end of the game, and you can tell him to hang around a bit longer to help out. If Tevinter was really as evil as people here are making it out to be, there would be no diplomatic relations with any of the nations (think like modern day North Korea, where apparently only Dennis Rodman can meet with the leader). Instead, all nations deal with Tevinter to some extent, and while they disagree with the Black Divine, the Magistocracy, and almost everything on a fundamental level, with the exception of Andrastianism itself, the Imperium is still in existance, over a thousand years (or thousands? I just got out of class and am pretty tired right now, so I don't remember exactly when it was formed, haha) after it was founded. This means that, as bad as it is, it's not completely evil.
Speaking of little of children, wouldn't Fiona and the mage rebellion's abandonment of Southern Thedas essentially put every future mage child at the mercy of either the templars or the kingdoms of Ferelden and Orlais, with no pro-mage military power or political power to defend them and train them. I suppose Fiona decided to cut her losses and leave them to their fate. Way to stand up for the rights of her own people.
I must admit, this is an interesting point that I had not considered. However, I doubt the mages would go to the Templars because the Ferelden and Orlais would not tolerate a rogue organization within their borders without oversight/support of the people (unlike the Inquisition, which has popular, religious, and national support). Also, I doubt the Templars would ever go back or be allowed back with the Chantry to re-legitimize them.
No Fiona's incompetent because she didn't do ANYTHING! The Vints showed up out of thin air (literally) and said "Hey a massive templar force is just outside your walls and they're coming to kill you, don't ask how we know, just trust us. All our help costs is ten years of slavery and your children fighting our war with the qunari, it's a steal." And Fiona went "Ummm, that's not good. Ok, you have yourselves a deal."
All she had to do was NOT take the Venatori at their word. Ask for proof, how they knew, as long as she did SOMEthing!
As it has been pointed out numerous times, Tevinter came in via time travel, offering support when it appears doom was approaching, after having hear it from disguised Tevinter mages who were posing as rebel/refugee southern mages. That means that, as far as Fiona was concerned, the source of the information was not Tevinter, but her fellow rebels.