Hi Paul,
I think you're just supposed to get the large majority of the scum. I would think the first 30 minutes but I've never timed it.
I always cover my chicken broth. It's supposed to be more concentrated if you don't cover because it reduces the quantity of liquid. But since she didn't say to cover the pot, I am assuming she meant for you not to.
The definition of simmer is to barely see little bubbles moving.
I have never made NT broth though. I just got my book a week ago.