First of all, I'm so sorry for you loss.
I haven't had time to read all the posts on this topic, but I'm suggesting the mare might have died of natural causes, and happened to be right next to the fence when she went. The fence may have nothing to do with it. Horses do have heart attacks. And do die of things we just don't know are going on inside them. Just an idea. Sometimes we never know the cause, even with a necropsy.
Again, my sympathy.