Can't change song info / songs missing from iTunes Music Library.xml

I cannot change individual song volume, start and stop time (and perhaps some other custom settings) for many of the older songs in my large iTunes library. I can go in and make the changes, but they don't stick. Immediately re-opening the Info window for the same song show that it has reverted.

After comparing "iTunes Library.xml" with "iTunes Music Library.xml", I think that the problematic songs are all missing from "iTunes Music Library.xml". Perhaps this is the problem? This has been going on for quite a while, so I don't want to simply revert to backup copies of the library. I also don't want to lose all my current playlist information if possible.

1. Should every song in "iTunes Library.xml" also appear in "iTunes Music Library.xml"?

2. Can I simply copy the entire Tracks block from "iTunes Library.xml" in a text editor, and replace that same block in the "iTunes Music Library.xml" file? I naively thought that might fix the problems.

Running iTunes 12.2. The music library originated in iTunes 11 ( or maybe 10; I can't remember).