Ok, I've got the firing pin "mostly" cleaned up, I say mostly because there was a small pit that I wasn't sure it was necessary to sand down and I'm worried about taking off too much material. Everything is smooth, and free from sharp edges, so I think it should be fine?
I'm not sure sure how to ensure the striker flange is contacting the rear of the bolt head when I check for protrusion, short of disassembling the bolt, which, from what I've read, should be avoided if possible. If I turn the cocking piece counter clockwise (as viewed from the rear), it allows it to go forward a bit, and the firing pin sticks out of the bolt head. The amount it sticks out is similar to the photo you've linked. It did not change much from the before to after the sanding. But I'm not sure if that is a valid way to check.