You should go and see a dentist or a periodontist. There could be many different causes — it can also be a hormonal change that’s affecting your body. Continue with...
Yes; because of the zipper, it will be easy to replace stuffing. Just unzip it and take out the old stuffing, replace with new stuffing and zip it back up.
Yes, you can install a regular zipper with a normal foot. However, if you want to master invisible zippers (and do other things like sewing on lace and sewing over...