This is all based on my observation, so someone please correct me of I'm wrong, but:
As far as I can tell auto-reverse just automatically applies MAX REV once you touch down, regardless of your IRL throttle position. It may require weight on the nose gear, but I haven't actually tested that.
The reverse detent setting controls the point in your throttle axis where the throttle transitions from idle to reverse (so if you have a throttle with a physical reverser detent, you can make sure the in-game throttle matches it). IE, if it's set to 0.2, the in-game throttle would transition into the reverse range when your IRL throttle is at 20%. If it's set to 0, your in-game throttle won't go below ground idle.
Disabling ground range prevents the in-game throttle from going below flight idle, regardless of whether you're in the air or on the ground. Vice-versa for enabling it, so if it's enabled, you can go into reverse thrust in-flight.