It wasn't necessarily a forgone conclusion though, sure the Templars thought they would crush the mages in a matter of weeks but it took a whole year of fighting for the mages to lose. I agree that you have to be held accountable for your actions but why does that mean Fiona must take the blame for what Lambert did? She thought she was doing what's right, sure the response on the Templars was predictable but does that mean you shouldn't do it because you fear the backlash? Lambert didn't have to attack the mages and start a war. There were a myriad of options he could have taken, but instead he chose to crush the mage rebellion with force. Fiona may have provoked him but you're always accountable for your own actions, so if you declare a war with Mages, regardless of the provocation, that's still on you.
Even if the mages had won, which let's face it, was pretty unlikely, but even if they had won, what then? They couldn't have overthrow the non-mage royalty of all the southern Thedas nations, so they still would have had to live in places ruled by non mages, so they could either have ;
1. Gone to Tevinter en masse, if the Imperium accepted them, then they would have, in no time started dabbling in Blood magic and other evil sh!t like that. (some mages in Fiona's group didn't had any problems with Blood Magic) which would have made them no better than the dangerous malificars the chantry warns the masses about.
2. Fight till either every mage is dead, or only the mages are alive (the former is more likely) because they would have never been accepted by the common folk or the nobility alike, especially after they annihilated the "faithful" templar order.
I say it again, fighting against tyranny is an admirable goal, but when thousands of lives depend on your shoulders, you have to consider your actions more carefully. And if 'fighting the power' results in an unending cycle of death and destruction, then it would be better to wait and bide your time for a better opening, rather than martyring yourself and your charges senselessly.
Agree with you about Lambert, though, that man was nine kinds of crazy ....