So very sorry that you are being "haunted" by this situation yet again. Having been to your farm I can assure the others that you are in the hill country of TX and there is just no way for you to easily patrol that farm!! So many areas where you cannot see a dog if it were there and I can feel that frustration from here.
You are right, for now being able to bring the mares up at night can help but it is a band aid, not a cure. But, new fencing all around?? Oh, my!!! Sad part is, if they really want in they will still find a way -- may take longer but, can happen.
Hopefully they will catch a few very soon and end the problem. Until them my heart goes out to you and what you are having to endure. I know how very much you love these animals and the great care you give.
Wish there was some type of "predator alert" every one could rig along their fences that would tell us when & where, they were. So many have similar predator fears, although you have had the most repetitive.