I would doubt the frizzen needs to be hardened on a new rifle. Instead of lead you can also use leather. They sell flint wrappers or you can make them out of some old leather gloves and such. The reason the flint broke the way it did was it probably was not tight in the jaws, and it slipped when it hit the frizzen.

You can also do as frontier gander said, flatten a roundball with a hammer and wrap the flint with that. On your Agate, try simply turning it upside down. Sometimes the way the bevel on the flint or agate is makes a big difference in the length of the spark and the shower of the spark.

Perhaps you are still flinching a little being new to the flintlock and that is causing the poor accuracy. Start your shooting closer if possible. I like to start at 25 yards. Good luck with the rifle.