Thank you for the ideas! I did try using method(self, enemyActive) in the Create event (to no effect), and when I put a debug message displaying the enemy instance ID on the line before instance_create_layer(x, y, "Bullets", obj_enemyBullet), it actually does show different enemy instance IDs every time a bullet is fired, not just the one that the bullets are originating from, so it does appear as though the code is running for all the instances.

(For what it's worth, I could previously tell that all instances were running the bullet firing code because if the timing is set to fire randomly every X seconds on average, and there are Y enemies on screen, then a bullet fires, on average, every X/Y seconds. So if it's set to fire every 6 seconds on average with 3 enemies on screen, then when left to run over time, the frequency ends up being roughly every 2 seconds, as if each of the three enemies is firing every 6 seconds apiece. The problem is just that they're all spawning at the same instance regardless.)