If Miller is correct as to why chilled is harder, and I have no doubt he is, the chilled would be harder all the way through. The fact that it is alloyed in the molten state would mean the hardness would be all the way through. Even though the chilled is a bit harder than dropped, it does not approach the hardness of premium, or magnum, shot which has an even higher antimony level. Note that your Hornady shot is "standard shot", as opposed to premium, or magnum.
As I understand it, the only shot that would be harder on the surface than in the middle would be plated shot, like your Luballoy or nickel. Problem is, today, it is very hard to get your hands on true plated shot. Most of it is just washed with nickel or copper and the skin is so thin as to not help that much, at least not nearly as much as the true plated shot.
You have a nice supply of some hard to find shot.
SRH
