RAW, even if you wanted to have them attack different targets it'd take a bonus action from you per directive.
You're right about the fireball; it's unlikely that all of them would fail a DEX save.
I like the idea of 4 attacks at 2d4+7 that scale down as it loses HP; may as well just have thresholds at 150 HP, 100 HP and 50 HP where it goes from 4 to 3 to 2 to 1. With nice round numbers like that it's not really any harder than keeping track of whether it's above half or not.
I got around to doing the math, and I think you could get pretty close to the statistical properties of the individual objects by using a swarm that autohits and does 10d6 damage (with no modifier). The variance of that is now well below variance of 10 separate attacks, but not zero, as it would be if they always did average damage, which is a consolation prize for giving up the positioning and AoO ability of the individuals, and the average damage is the same as the individuals against an enemy AC of 18. Then at HP thresholds of 160, 120, 80, and 40, reduce the damage to 8d6, 6d6, 4d6, and 2d6.
If you really wanted to get fancy, you could add a constant modifier (which can be negative) of c * (18 - AC), where c = 3 at full HP, which will keep the average damage tracking roughly. At 120 HP c goes down to 2, and at 80 it goes down to 1.