Hello from a fellow freak. Not only in the nail biting area but many of the other behaviours you talk about. I stopped “biting” my nails when I was no longer studying, but that doesn’t mean I stopped putting them in my mouth. My nails are very soft and break easily but I can manage to keep a few millimetres growth on them. I always have them in my mouth. I eat anything that happens to be under my nails. Even Covid hasn’t stopped me putting them in my mouth, but some hand sanitisers taste disgusting. I had acrylic nails for 15 years and always had those in my mouth too. I had them taken off when I travelled. The downside of always having clean nails is that I often pick up gastric especially when travelling.