In your Import configuration under tasks, "Write Files tags ...." should be unchecked
In your import configuration, you might have added rules that are linked to your watched folder (click edit). If no rules were added by you there shouldn't be any.
uncheck General=>Online Metadata=>Allow Auto CD Lookup/Submission (Even if this is checked there would be a popup asking for confirmation
Unless there is some peculiarity about the Mac version I'm unaware about, JRiver doesn't just overwrite your tags or move files around unless you tell it too. It sounds like you reimported your media into an empty library, so its probably in your auto import config somewhere.
What you should do if you were happy with version 20, don't even bother trying to figure it out what you did.
Just take a v20 backup file and restore it in v21. Your old settings will be ported over and your original tags will be back. Usually youkeep the older version on your machine during the upgrade and everything is ported over automatically. If you have removed version 20 before installing v21, then you can still restore the backup to accomplish the same thing. Library backup files are done automatically, so short of nuking your machine you still should have it.