I'd like to refinish the black into some type of FDE. Who does good reasonable work?

Krylon works for me

I don't fully understand the nuances of air cured versus oven-cured cerakote, their relative durability, and whether you can bake a USP frame or not, but there are tons of places that do cerakote. This sounds like simple-enough a job anyone can do it. I know of 3 places in my area. Start calling local gun shops and see what they know. If you're willing to drive to Louisville KY Black Dove does excellent work.

Talk to Matt at LRK Mechanical. He does wonderful cerakote work. I've seen a tiffany blue 320 module that looked as if it was factory molded that way and not a cerakote job, which it was.