[Zeke]

I understand the need to not damage the bearings. You didn't actually tell us about the bearings —ball, roller, thrust, etc. You can back the sprocket up so that any blunt force will not transfer to a bearing race. Heat, not heat, penetration oil, all of it, you are trying to break a bond. A preferred method is to disturb the bond by shocking it in the opposite direction.

If you go gorilla on that key you're likely to pull it in half with half down inside. Also, tap it up and down, keep shocking it. Always works for me. And unless you turn the sprocket to yellow or blue while heating, I don't believe you're altering the temper. Might want to wire brush the thing bright so you can observe. There's also the trick of turning a compressed air can used for computers, etc., upside down and cooling the key and center shaft.

That key looks pretty stout and you might be able to drill a hole in it. Why? Well, more surface for the freezing technique and an additional pull point. The tools shown are pulling along one edge essentially helping to wedge it further. You want to pull along the centerline. If it's coming out, you won't need to pull the hole apart. It's lots of shocks and minute movement more than pure constant tensile force.

Even another idea is to use the wedge tool and tap the key towards the outside. You will be triangulating the pulling force