In the case of the charr army, they'd seen what the Searing had done to Ascalon and weren't willing to risk that happening to Orr as well, so they marched out to intercept...
If you know the enemy is carrying a limited-range nuke with them, going out to intercept means that you at least have a chance of stopping them before the nuke hits the city and people you're trying to protect.
Regarding control: One possibility is that the Titans neither needed nor wanted the Flame Legion shamans to actually understand what they were doing, and just gave them enough information to use it as a strategic nuke without giving them the instructions for the tactical nuke option. Remember, the titans had their own agenda in which they didn't care one bit for the charr except in their usefulness as tools. Another possibility is that the Pact is simply showing its hubris - the GW1-era shamans may have known quite well how to use the Cauldron in a more targeted sense but chose not to (after all, from Pyre's dialogue, the 'scorched earth' result was part of their strategy).
Regarding how it's powered: My impression is that it IS a battery. Power comes from somewhere else (possibly the sleeping Kralkatorrik, although this was never fully confirmed), is stored in the Cauldron, and is released in the form of crystalline meteorites. That's why the Iron Marches Cauldron is only able to Sear a single village.