great ideas!! I heard that a heating pad on low and covered in blankets works well. Just make sure that the heating pad stays covered if uncovered it might burn the little one. The potty thing is so often overlookedYou should put her in a box with a heating source. It's really, really not safe to sleep with her. I don't want to scare you but she could easily suffocate. You can put a sock with raw rice in the microwave for 5 minutes. It will stay warm for 3-4 hours and the kitten can snuggle it.
Are you stimulating the kitten to potty?