Thank you for the reply. Here is some additional information that may help others, if they're having similar problems.
Here is a list of software I use (other than MC) that is known to change tags and/or file names:
MusicBrainz Picard: yes, agree this is one of the more problem-causing softwares. Pretty sure it will not change a file's name at least by default. The "scan" (analyze audio) function is occasionally used. However in experience I find a high number of false positives so use it with caution.
Testing idea (for me): Feed it good examples of files I find screwed up, and see if it creates problems.MP3gain: I've never noticed any tag or file name issues.
MP3Tag: This is my "go-to" tag editor and yes it can screw things up. I routinely use it to preen, correct and (yes) rename a file based on its tag information.
BeaTunes: very rarely used; this will analyze and store its own tags into the song files.
Some observations:
1. Generally I am familiar with the music and the files I am manipulating. I proofread and "proof listen" usually, before adding to my permanent collection (\KEEP).
When I add a file to KEEP it should have been vetted by me. It is extremely unlikely that I would make this many errors, if human error were the issue. Once added to a permanent collection, a song shouldn't change its name any more than an oatmeal cookie left on the counter overnight should turn into a head of lettuce (stupid analogy I guess).
2. Virtually all the errors I've found are correct artist but wrong title and often other tags. This
implies software error. Since sometimes a different album's data is used, this implies error occurs when operating on multiple albums (one of my editor programs.)
2.5 Re the "update library from tags" comment: I easily understand that MC would import bad tag data from files. But under what conditions would it change correct file tags? Only when the tag editor is used? Does it ever change an actual file name, even at user's request?
2.5 I do occasionally delete files in MC (shift+delete menu); however, almost never with KEEP. I also know that tags can be edited in MC, but again, I rarely use the feature, instead use MP3Tag.
3. [Succinct summation of this issue?] Files in KEEP in theory are perfect and do not get edited by manually controlled editing software. File names and tags change unpredictably.
Sometimes to song data for songs that were never added to KEEP. If this is correct, it tends to exonerate editing software and casts suspicion upon the media player, which unfortunately is MC.
Actions I will take to try and further isolate the issues:
1. I will compile a list of the errors I find, stating briefly what's wrong (and right) perhaps.
2. I will store the "bad" files in a separate directory, for possible later analysis.
3. I will consider additional suggestions offered here.
4. Stopgap ideas? Make the files read-only? But even that will not protect against modification or deletion by elevated privileges.
Addenda:
(At risk of repeating myself. I am well into middle age so consider it practice for my old age!
) I will systematically listen to the 5000+ songs in KEEP, I'm familiar with most but not all. Already I've found several errors. These I'm collecting into a directory for further analysis. I'm also keeping a log of what errors I've found and steps in testing.
This is preliminary: I've run both BeaTunes and MusicBrainz on the sample songs.
In all cases the songs were correctly analyzed and identified (artist, title at least) For the Collective Soul case, this included obtaining a copy of the entire album, stripping out ALL the tags (with MP3tag); MusicBrainz scanned and correctly identified all songs. Similarly, BeaTunes had no trouble scanning the files and updating the correct information.
NB: MusicBrainz
does have the ability to change a file's name, at least per the docs. To the best of my knowledge, BeaTunes does not ever change a file name.
I will do additional testing and report. Thank you.