Now, about the situation. 20 cows with a 250 HP attack kill 1 angel and wound another. And with their gaze, they can kill another 200%, i.e., 2 enemies.

What difference does it make how many they kill with a simple attack? It doesn't change anything. Where does the 200% figure come from? It feels like I'm wasting my time writing all these mathematical calculations. You try, you explain everything, and then someone tells you that 10 cows is 100%.

, and a stack of 10 cows has a 100% chance of doing that.

Where does 100% come from? They won't hit every time. If 100% and 87% are the same for you, then I suggest the following. Put 1 bullet in a revolver, spin it, and shoot yourself in the head. You'll have an 84% chance of survival. If that's the same as 100%, then you have nothing to fear.

Now, I'm quoting FizMiG:

When attacking, each of the attacking Gorgons, in addition to the direct damage, has a 10% chance to instantly kill one unit in the enemy stack.

I highlighted the key word in red! EACH!!! This means that the calculation is separate for each cow. I understand that this is difficult to grasp, so here's an example from real life:
Four people are standing with coins. Each has a 50% chance of catching an eagle when they flip a coin. This does not mean that the four of them together have a 200% chance of catching an eagle. There is still a chance that they will all flip tails.
If, even after this, 10 cows have a 100% chance, then I will not write or prove anything else.