When our avatar, Nikki was a kit, she'd lay in her hay rack, again, on her back with all 4 legs sticking up--she was a sweet but very strange little bunny.
So sorry for your loss, but it serves no purpose to blame yourself as we have no time travel ability nor do we have a "magic wand" to change things. We are granted such a limited time with our loved Buns no matter what we do. We have rescued 60+ buns and re-homed a lot, but most stayed with...
I've always felt that "quality" of life is more important than length--fortunately our little "velvet" bunny had both. We have had some with us for only 3 years before passing to the bridge but we did our best to make sure they were great years. Still miss you a lot my little one, goodbye for...
We use child gates as a barrier and have put 2 together so it is 6 feet high--none gets over. Your bunny will never change the behavior, so you will need a better barricade.