I use it before styling - the disadvantage of that is that you will get some of the excess Toppik on your hands but you just wash that away. If you use Toppik after styling you are going to need to move your hair around to get it where you want which means you probably messed up your hairstyle. I use it around my hair line where I have hair, not in front of hairline obviously. I guess you could use it a bit further in to avoid it getting on your forehead if you drag your hand there when sweating. I have yet to have issues with it running down my forehead because of sweat or rain (no rain storm though, lol).
You can most certainly drag your hand through your head unless you drag your hand against your scalp.
If it feels like hair or powder? Uhm... not sure, I don't feel it in my hair.