You didn't like my response last year about painting nails as a business, but I'll answer this anyway: you may run afoul of the law if you sell a product applied to the body. With cosmetics, this might only happen if someone is allergic to your ingredients and sues.
FDA Authority Over Cosmetics
If you go ahead,
1. I don't think it's a hobby business. You aren't 'crafting' anything really. You are assembling/mixing a product, not building or sewing.
2. No. Consider the name you choose a copyright by virtue of having documentation that you started it on a certain date.
3. Taxes are easy. Save every receipt for next year and worry about it then. What you do need is a state sales tax number. Contact your state for that.
Of course ebay and etsey are havens of homemade products. I have sold thousands of dollars worth of my and other's things on ebay. Here is a nail polish maker on ebay:
LUCKY POT OF GOLD Shiny Gold Glitter Handmade Indie Top Coat Nail Polish Lacquer