How do I roll back to previous export for a batch/whole catalog of synced photos?

Just a thought- Not knowing -'How you edited in B&W' is it possible that you could-

InGrid View, Select all, Press keyboard [V] ?  [V] toggles Color<>B&W.

In the future, consider the use of virtual copy's, and exporting to modified file names.

hmm, so edited for color first, then did you add additional edits to go B&W? And as such, the history includes all those edits all the way to B&W? If so, consider creating Virtual copy's of the current state (B&W), then edit the non virtual ones as to remove the steps at the top of the history that converted them to B$W. Simply (unfortunately one at a time I suspect) Click on the history state before the conversion to B&W and forget (clear) the steps above. Do not have any form of sync set. Do not have any of the virtual copy's selected. Test first.

If you backed up the catalog file before changing to black and white, then that's what you would use. Otherwise, I think you have to undo the editing one photo at a time.