Tevinter is in no position to attack Southern Thedas and even the Qunari would eventually have been driven back by attrition. That said, although they probably would have won in the end, if the Mage-Templar war was that one sided, it's unlikely there would have ever even been a conclave. The templars would have had no need to agree to negotiate for peace if they had already all but crushed the mages.
Given Fiona and the mages context, the Conclave was because the Templars had all but crushed the mages. The caveat for the 'but' was the mages holed up in Redcliffe- ergo, under the protection, and authority, and influence of a chantry-aligned Kingdom. The Templars wouldn't attack (lest they pick a fight they could not win), but the Mages couldn't continue either.
Politically the Conclave was up in the air, but functionally it was a negotiation of the mage surrender. That's not as bad as it sounds politically- the mages had a good political hand and the Templars some significant restrictions to get a victory- but the war was pretty much over by that point, since there were only four broad strokes of what could happen.
The Templars alienate Ferelden by attacking the last mage bastion- probably leads to the Templars being destroyed. The short term victory over the mage rebellion ends with political failure because the Templars won't be in a place to re-instate the Circles and run them going forward. A Templar attack is a fight they can't win (against Ferelden) and a lack of political involvement going forward (restoring the Circles).
The Mages can't alienate Ferelden too much because doing so would see them kicked out of Redcliffe. That either leaves them with no other defense against the Templars waiting outside the gates- the whole reason they needed refuge in the first place- and thus total defeat. Having alienated their patrons, and with no other survivors to guide the circles, the Templars lack a real opponent to restoring the Circles.
The Mages and Templars can come to a peace treaty at the Conclave. Specifics would depend on context and deals, but outlines would almost certainly include a re-establishment of the Circles, some face-saving reforms and limited immunity for rebel mages, and likely changes to Chantry heirarchy in favor of the Templars (such as the replacement of the Divine in favor of someone the Templars and Seekers would respect and follow).
Or the Mages and Templars can NOT come to a deal... and end up in a status quo in which they wait on opposite sides of the wall until someone ticks off the major players enough, or the status quo becomes a de facto deal in and of itself. Redcliffe becomes a de facto mage circle/college, but mages can't leave. Mages have freedom from the Templars, but can't oppose the Crown's authority. Either it works- becoming a status quo- or it doesn't, and either Templars or Mages get Ferelden firmly on their side.
No matter what, the mage rebellion's ability to continue on its own power is over. It's dependent on the favor of Ferelden.