I wish to create precisely 15 second clips from a music session.
I can set an in point, then set an out point 15 seconds later on the timeline. However, ideally, I would like to have a precisely defined 15 second gap between in and out points, and I would like to slide the two points together across the timeline to preview various 15 second clips.
Not sure I fully understand what you are trying to do, and I read this several times...but here goes...
If you set up a template timeline with a black clip or slug and then blade (add edit) it every 15 seconds, you can then simply replace each section with real footage pretty quickly. There are several techniques you can use for that using a minimum of keystrokes. Use the slip tool to change the previewed area once the footage is edited into the timeline.