Then the fomorians attack and the changelings don't assist, at least not until they become aware of the attack, I assume. I don't see why this would stall the game.

Did you ask the players why they're reluctant to provide this information? Maybe they believe the changelings have some alliance or other arrangement with the fomorians, and want to avoid causing tension with a group whose allegiances they're unsure of. Could be any number of reasons, depending on specifics of the game world.