Annealing and then rehardening/tempering the frizzen is definitely NOT the "easy" way, if you don't know what you're doing. (and even though I do know what I'm doing, I'd still definitely rather just stone the front of the frizzen off smooth!)
What is it that you don't like about your patchbox spring catch? By the way, it's something that you'll have to bend, shape, and fit to your gun. When you buy a patchbox latch, consider it a "blank". Although the "drive-in" front ends can be a pain, this type of latch works well, it's simple, and it is THE 18th century wood patchbox latch. You are simply not going to find any American rifle with a wood patch box with a push button latch (and you would be very hard pressed to find a German one that did.) Anything other than the ordinary type latch would stand out like a flashing red light.... and waving flags, and silver sprinkles...
Also, I have to say that your patchbox lid is still way too thick. Especially on such a slender gun. To bring the thickness down will likely require a new brass end, since you will cut into where your screw heads are. It needs to be fitted better to the surface of the buttplate too. :wink: