Place it thick enough under caps so they self level with some tapping. Clean out any that oozes out before it sets. Not sure why you would worry about the heat as stone will take a lot and you have 4 feet which should be adequate. If you can stand 4 feet away surely the wall can

We'll use the thin set mortar mix, as you suggest. Makes sense. I only mentioned the heating cement out of ignorance. I read a DIY article, and she was adhering capstone to an actual fire pit. In her case, it was necessary. But I wasn't sure of the rules of distance. We obviously don't need it. Thank you again.

We're almost finished. We just need a few rain days to help clean up the stone so we can be sure we have the ones with the best marbling. We washed them down, but they're still dusty.