How long do you think we have to dilly-dally? Sure it was all well and good so long as we (kinda) knew the sorceress had the amulet, and where the sorceress was. But if she hasn't pawned it, she might be seeking to do something with it- like drain it. It's been two days now, celebrations can only last so long, depending on the disposition of the partier.
So, time may be in short supply.
As for fires: we don't want to be caught, most likely outcomes here are death, vivisection, or a return to slavery. We're also a dragon: what are puny humans compared to us? Sure, at the moment we've not reached our potential, but we should have pride in our Draconian heritage.
So, given that we don't want to get caught, we don't care about human life, and time may be of the essence, what's a quick, sure-fire (pun) way create a massive distraction? Setting half the city ablaze. It also has the added benefits of A. Not consuming our mana & B. Turning the city into a giant mana font.
If we're as awesome as we should be, we'll pull it off like a boss. Even if we fail, even a full-grown dragon would respect the damage we caused & the independence we showed, (and they're the ones that matter, they're the ones that won't enslave, vivisect & kill us; unless we act like a despicable welpling at least..).
Purple prose aside, I think it's risky as hell, but that fortress looks rather impregnable & I'm inclined to take action rather than wait around. And I think it could work out rather epicly pretty well with some preparation.