Your helmet is fine, Krylon Fusion is fairly forgiving. The only indication that you will have messed up is if you get a web like cracks in the paint.
The reason it tells you to apply a wet coat over a wet coat is for adhesion, but if you do it too soon the propellant doesn't have a chance to evaporate and that can lead to all sorts of problems . Paint sticks better to partially dry paint, but there won't be any problems doing another coat later.
Not trying to derail, but has anyone tried Krylon H20 yet. I know a few Vaders that have used the H20 and the results are mind blowing, but didn't know what kind of finish the white would yield.