“100% USDA Grade A milk is not permitted to contain anything other than cow’s milk.”
If you are telling us that we should not be concerned because we do not want or like horse milk or treifa milk and that if there were any, it is bitul, and even if we were to buy a carton where it’s not nullified, we acted correctly by buying USDA certified milk and it has a presumption of being kosher and we go by the majority since the very vast majority of cartons are fine, I agree with you. I do not agree on your remark of lashon hara / motzi shem ra — which, however, you do not qualify as slander, thus it appears you too think there is truth to it.