They were already thinking in those terms - they knew they were almost out of food, we'd had the whole desperately-scavenging-the-hills-and-town scenes in a previous episode. They were entirely aware that if they allowed themselves to become trapped in the tower they'd starve.
So it's lucky this isn't the real world, then. Honestly, are you expecting real-world engineering solutions for triggering volcanoes? Those aren't a thing.
Huh? Adar had recovered the hilt, and the last thing he did before the cavalry arrived was to instruct Walgren to do something for him. Then he rides off with the fake hilt as a distraction, providing Walgren the perfect opportunity to slip away.