I don’t know about the heater.
The hutch must be raised above ground, preferably 3 feet or more. There must be a space where they are sheltered against wind, rain, snow and direct sunshine. Preferably that space should be isolated with styrofoam. The netting/cage bars must be predator proof, i e NOT chicken wire, all netting must be welded and the “holes” preferably not bigger than 0,5 inch times 0,5 inch. Weasels and ferrets eat rabbits. Rats can hurt them, but they’d probably be attracted by their food rather than the rabbits themselves. Mice are probably not possible to keep out, even if the hutch is standing on legs, as it should be. There must be a roof, house-cats, weasels and foxes etc climb, prey birds come from above.
They should have bedding like straw and fleece blankies so they can “nest” and they will need a bit extra food to keep warm and grow thick fur. When it’s below zero you need to refresh their water twice a day because it will freeze solid.
I can see how this is distressing for you, but I hinks it’s better than to have to get rid of the buns. Since they are two, they won’t be lonely and they will help each other keep warm. The main worry I think would be the transition to outdoors from indoors in the middle of the winter. How cold does it get in Pennsylvania?