You know it's a case to case basis. The sedation is as it is described given to sedate an agitated or unsettled horse. Not all horses require that extra injection prior to being euthanized.
When they are overdosed on the medication, they don't even know it is happening. Yes they do drop to the ground and it is quite difficult for the owners to see BUT to the horse it never happened as to by that point the hypnotics have already long kicked in. The same thing would happen to humans if we were going thru the same thing. You can compare to us going thru anesthetics-there is a good reason why they lay you down when they give you those meds.
Sorry to hear you went thru this very sad situation. I think it's every horse owner's saddest day. Hope the rest of your day and week gets better, keep your chin up