in a revolving slot

I am guessing it's referring to the opening act in the tour not being one musician or band, but a selection of them.

So, one of those people was Waylon Jennings, according to the passage. Let's say for sake of an example that there were two other possible acts, The Lumineers and Amy Grant. Never mind who was really there. This is just illustrative.

If the tour went on for a long time, then these other acts might not always be available. This could be for anything from the wedding of a friend to a dentist appointment, or just wanting to rest up and be at home and not tour quite so much.

I like this idea, and I think it's a smart one. Metallica gets a variety of acts to open for them, and the people in those acts don't burn out as quickly because they get time off.

Thanks a lot. Does the part "who James admitted" happened simultaneously with Waylon openning for Metallica, or does it mean that James had admitted before? Or he admitted many times before the concert with Waylon AND after. Thanks a lot.

The part about "who James admitted" (it's not worded well) is that James admits that Waylon Jennings was the inspiration for a particular song. Nothing to do with the concert, so far as I can tell.