Why are some reverb controls not editable?

I can't reproduce what's in your video on a PC, so I'm wondering if it's a Mac thing. It's quite normal for some controls to be blocked when playback is taking place, simply because everything would grind to a halt if they were altered - but they certainly all ought to be available when the transport is stopped. I couldn't find any difference in behaviour between reverb applied to the original track and reverb added to a bus track either - they both behaved identially, and it didn't matter what reverb it was either - the peformance was consistent.

That said, there have been some strange things happening with Multitrack view lately. One thing you might like to try is downloading the beta version of Audition from the CC app and seeing if the same thing happens; some aspects of multitrack performance have been improved.