I'm not appealing to anything except her own words. In the mages path, she shouts at Alexius that he cannot involve her people in this. She clearly does have a line she will not cross.
Except he can, and she does- just as she's crossed numerous other lines she's drawn in the sand, which by your previous argument she wouldn't have because it would have been stupid to do so. But she made them too.
Fiona was under the impression that Alexius forcing Art Teagan out was only temporary. That the mages would leave and that would be the end of the conflict. Arl Teagan's ride to the capital meant that there would be a deadline for their departure. So Fiona wasn't too broken up by that. Her explanation to Anora (and/or Alistair) was that she never intended for that to happen. But ultimately that was out of her hands. She couldn't refuse Alexius at that point. That was not her strategy. Alexius is the one who forced Teagan out. I doubt Fiona agreed with that.
This is a great argument for why Fiona is a gullible dupe who wasn't in control- but not why she wouldn't go along with an allegedly monumentally stupid course of action when she already wasn't in control. Especially considering you've just spent the paragraph trying to defend her taking a monumentally stupid course of action, and conceding that she wasn't in control.
Haven was not easily overrun. Aside from having a frozen tundra separating it from any danger, it was also a well fortified and defended camp. A few trebuchets repelled an entire army, because the terrain favored the defenders. The only reason Corypheus' forces got through is because his red-lyrium dragon took down most of the defenses and laid waste to the inner campsite.
Haven was overrun in something like a single evening, which by the standards of Dragon Age era siege warfare is amazingly quick. Even Adamant had to be put to siege before it could be stormed. A few trebuchets managed to set off an avalanche, which helped... for maybe half an hour, before- as you concede- Corypheus squashed the defenses with something the Inquisition had no ability to resist.
That an avalanche caused significant casualties to an uncaring master doesn't really change that Haven got overrun in short order.
If attacking the Inquisition made sense, then they would have done it before the Herald returned to Haven. Heck, while the Herald was at Theirinfall even. But no, they waited until after the Inquisition gathered the templars and returned to their base. There was no sense in the free mages attacking. The only sense it made was for Corypheus.
If we ignore reasons why the Mages might not be mobilized to attack the Inquisition pre-Templar alliance, such as there not being a Templar alliance to justify pre-empting, what you say might make sense.
If we don't do that, your protestations of not having any reason, when reasons have already been provided, falls short.
No, because the mages would have left Redcliffe before the Inquisition returned from Theirinfall. Remember, the Ferelden crown forced the mages out of Redcliffe if the Herald takes the templar path. So the mages had already started heading north. If the Inquisition was really going to attack the mages, they would have camped at Redcliffe, brought in forces from Haven, and then followed the mages north. But they didn't do this. Instead they went straight back to Haven and then closed the breach, just as they said they would. So the mages had no reason to think the Inquisition would attack them. Fiona had no reason to think this.
You make assumptions of the timeline not justified on the non-existent timeline of events. Theirinfall is on the far side of Ferelden. Redcliffe is not. It takes considerably longer for the Inquisition to return from Theirinfall than it takes for the mages to muster from Redcliffe. There is no reason to believe the mages begin marching from Redcliffe to Haven before the Inquisition even finished returning from Theirinfall.
Nor do you provide a convincing argument of 'would' as a substitute for 'needs to' or invalidates other campaign strategies, or provide any answer for why the Templars wouldn't be willing to address the Breach first as part of any anti-Mage Inquisition-Templar alliance. You claim that because the Templars didn't make the Mages the absolute priority in a particular marching order and lay siege to a Ferelden town immediately from across the country rather than stage from Haven- well, somehow this lack of making the Mages priority number one is somehow proof that the Mages couldn't be priority number two in turn- even though letting the Mages be priority two could bolster the Templars by having them gain political support in their campaign after, you know, saving the world first.
The fact that Fiona doesn't have control of her own information sources- so much so that she hilariously thought she had to sell her people into slavery to mages in order to save them from a non-existent anti-magic army thanks to a whisper campaign by her master even before he was her master- is also completely ignored.
Nothing justifies mages trying to attack templars. Mages are weak against templars. Their best strategy at that point would have been to fall back to Tevinter's borders and let the Separati take the templars. And again, this would all take place in Ferelden, which would of course make enemies out of them as well. Bad strategy for the mages, not so much for Corypheus, although he played a foolish hand in his own way. But the point is, Fiona would not have gone along with that willingly. Alexius was ousted, and Fiona could then see the truth. She was never working with Tevinter, just some cult that was commandeering her movement.
Mages aren't attacking Templars for no reason. Mages are attacking the Inquisition, who is in the middle of consolidating an alliance with Templars, and who is in a position to interfere with any hypothetical retreat to Tevinter.
The Mages already had abysmal strategy, and had already made enemies of Ferelden- back when Fiona stood by and allowed the Arl to be ousted. Being ousted by what she thought was a foreign power doesn't make it any smarter, or any wiser, when Tevinter has no means to reach or extract the mages even if they wanted to.
Fiona is an incompetent revolutionary, and has been from the start- she's spurned alliances, betrayed willing allies, failed to prepare, and couldn't even keep control of the mage rebellion even before she ousted herself. Under her leadership maleficars were running rampant right outside her gates, and the Mage Collective decided to leave her behind while the Adults talked politics at the Conclave rather than take her along.