With the same ease and extent that mages can? No.
Gaspard is the Emperor of the most powerful Empire in the world and that makes him extremely dangerous.
But he could never create the darkspawn or open the Breach.
And a skilled Chevalier would, eventually, have been overwhelmed by Redcliff whereas Connor, a child, destroys it by himself.
The Breach which was opened thanks to the power of a god and the darkspawn which took 3/4s of the resources of a world-spanning empire? I doubt most mages could accomplish the same. But give me the power of a god or a world-spanning empire and I could crush the world. And it's true about a chevalier, but there are a lot more people in the world than mages, if a small proportion of each population went mad and killed people you really think the body count wouldn't be the same?
This is not a concession. A concession would be dispose Fiona from her title, help the templars to put down the libertarians.
The restrictions would fade away, you forget why they were imposing them. Anders and 2 assassinate attempts against the Divine made people fear mages even more, before that the restrictions werent so hard (with the exception of Kirkwall).
Justinia shouldnt have betrayed the templars , she should have talked with Lambert to free the First Enchanters but should have executed Fiona for what she did (and sometime later send a Assassin to kill Lambert, when things have calmed down).
She did take the position of the First Enchanter, she was voted at young age even.
She doesn't have the authority to depose Fiona, that's the Circle's job. I'm sure that once the vote was concluded in Wynne's favor and the reforms put into place Fiona would have been deposed properly by the Circle. Heck Wynne might have been the next Grand Enchanter. Reaching outside her authority and ordering Fiona deposed would have exacerbated the conflict by convincing the mages that the Divine wasn't interested in their welfare or respectful of their rights. And the Divine was trying to prevent violence, how does putting the libertarians down do that? It only makes the Aequitarians think that the Divine wasn't interested in peaceful reforms and, like the Templars, is more interested in crushing dissent.
The restrictions are irrelevant, they were not the reason Wynne or the other Aequitarians were looking for reform.
I imagine her talking with Lambert would have been the final straw that Lambert would have used to rally the templars and try to get the Divine deposed, and he likely would have said no anyway. And as I said above executing Fiona would have just prompted even more discontent because Fiona didn't actually do anything worth execution, she proposed a vote and she did so during the conclave where mages are technically allowed to discuss anything. Execution would just have been another sign of the Chantry's tyrannical rule and desire to stamp out sedition.
And who says an assassin would have actually been able to kill Lambert? He's the Lord Seeker and lives in a military establishment filled with Templars. Far more likely the assassin would be caught and tortured and when he/she gave up Justinia the Templars would rebel. Assassins are not a fix-all, in these situations they're often more dangerous to the sender than the target.
Technically since the Circle is meant to be dissolved she can't be First Enchanter but obviously she is since the other loyalists still maintain the Circle.
Besides this conversation is pointless, we can't possibly know all the factors that led to Justinia and Leliana's decision. I must simply trust that they both made the best decision they possibly could because they're both supposed to be extremely good at their jobs and at the Game.