Thank you for your reply. Unfortunately, my original post was cut off and only the first third of my question was posted. In the other two thirds, I went over what I tried, which included your suggestion.
I also tried these without luck:
*Uninstalled RDC (using a third party uninstaller), and did a complete file search for anything with "rdc" to clear all related caches
*I tried creating two accounts, ran RDC on one, did a full folder diff - but did not find anything new.
*Deleted ~/Library/Preferences/ByHost/.GlobalPreferences.[MACaddress].plist to reset all mouse/touchpad settings
*Ran lsof to get all open file handles when RDC runs to see if I missed anything
I am unable to reproduce the issue with other user accounts, no matter what combination of button-switching I try. I copied my entire Library/Preferences folder to a new account - still cannot reproduce.
Incidentally, the mouse is reversed regardless of what the primary assignment in the Mouse setting pref pane is. That seems to be the issue - RDC respected the original change, but not the change back.
This issue has persisted through an OS upgrade and two time machine restores. At this point, the only solution seems to be to start over with a new user account - but I know that the setting is somewhere in my user folder, so I don't know what to leave
out!