Will this apply to the scalp that does not have any hairs? And stay put? Or will it only work if it has hairs to adhere to?

Yes, I have a bald spot from burns during childhood. Its about 1-2 inches in diameter and is very obvious. I use the Topix spray on the spot first, let it sit for a few seconds, apply the fibers and spray again. I've used regular holding spray/spritz when the topix spray ran out and it works the same and sometimes better. I am AA and I dont wash my hair daily so it stays up to 2 days before needing to reapply.

Is this messy? Does it get on pillowcases?

No. Is an excellent product. It does not stain the pillow cases. If some residue falls on the pillow case you can wipe it off with your hand. I have been using it for years and I hope that I can continue buying it in the future.

Can you style your hair with other products after using this?

All I use is Toppik colored spray to give the fibers a little hold. I won't use hot tools for styling with fibers in hair. Don't use gel or anything like that. It becomes a mess.