I do not at all understand your suggestions.
You say to try restoring a library backup. In my question, item #4, I state that I already did that.
Rename, Move, & Copy files does not seem to update tags at all, with or without the Find/Replace tool. It is a tool for renaming files, not changing tags. My filenames are all exactly as they were, except for having been moved to a different drive. I do not want or need to change my filenames.
More relevant seems to be to try to use the tool called "Update Tags from Library," but when I try, I get an error message saying, "You have disabled the option to allow MC to update tags." I cannot find anything in Options with a name like this, so I don't know how to follow the instructions to ""Change this option and try again."
Once more, my question is how do I get my data back into the tags? Data that I entered track by track for each album previously. Please at least tell me how to undisable the option to allow MC to update tags.
I note that in the option to Manage Library fields, nearly all the fields I use have the box checked to "save in file tags when possible."