'Start some fires'? The monarchs would be dead. All the nobility and their retinues at the Exalted Conference would be dead. The Divine would be dead. The Inquisitor would be dead.
Very armchair-general to call it "poorly thought out" or poorly-coordinated, when the only reason the PC didn't end up a red stain was Solas' sabotage.
Well those gattlock barrels aren't like the Venatori fire ship or anything. They're basically as powerful as a fireball spell from Origins. So unless you think Anora is going to climb to the top of a pile of gattlock barrels to sit on her throne, there is no chance she'll be blown up by them. Alistair? He'd probably mistake them for a new throne accessory or something and think they were cool. The Cousland king/queen? No way.
Oh yes, it would be quite the blow to Ferelden to lose Arl Marmalade. And Cyril? Just one member of the CoH? Big deal. And my Divine was Vivienne, who has survived... well... multiple gattlock barrels actually, and far worse. No, such an operation was doomed to failure. There's no way they could have blown up all the banns, arls, teyrns, kings, queens, emperors, empresses, and suitable replacements all within a short period of time. And if they had tallises in place to stab where their fires did not reach, or baaresad warriors to seize lands, then they would have done so even after the Dragon's Breath plan failed.
No, I'm afraid Dragon's Breath and the promised sweep of the south was nothing more than the insane machinations of an extremist Tal-Vashoth.
Just because Solas discovered the plot first does not mean the Inquisition wouldn't have discovered it as well without his help. Sera was already suspicious. Spirit Cole would have sensed it coming. Leliana... er... okay, forget Leliana. Maybe a Tal-Vashoth Iron Bull would have caught it. I don't know. The game seems to portray him as incompetent. I think Vivienne would have caught it too. She notices stuff like that because she knows how the Grand Game is played through servants. And my Inquisitor definately would have.
And let's not forget what this DLC was. Forced exposition that disregarded player choice and ruined various characters.