So I did a little more testing and discovered that the speed comping feature has (in my opinion) very odd behavior that complicates things. It behaves in very different ways depending on if "dim" is selected before speed comping starts or after.
Select dim before speed comping - Select the track(s) you want to audition and click shift+space. On each selected track the solo light turns green and we hear all the tracks with the non-soloed ones attenuated. CTRL clicking on a track will add or remove it from the "soloed" group as indicated by both the volume and the solo button. This is all quite logical and effective.
Start speed comping without dim enabled - You hear just whatever tracks were selected before hitting CTRL+space. So far so good. But "solo" is not lit on any track. Why? Now click dim. Nothing changes. All the non-soloed tracks are silent. Solo lights are still all off.
So the bottom line for me is - click dim first. Not doing so likely explains why some of us had trouble and others did not with this feature. But I can't help but wonder if this differing behavior is intended or a bug. Perhaps there is a logical reason for this, but it seems unnecessarily complicated to my little brain.