From my end I am very happy to report Nathan and his process (hacks and all) absolutely did the trick.
I have confirmed that the sign up dialog is now working, intending users can again select multiple answers from the drop down options, and can view and edit these again as approved users, and the old multi-select plugin was successfully removed.
Thank you to @nathank, @omarfilip, @simon and @RGJ for taking the time to consider this issue and share your knowledge and insights - more proof, as if any needed, of how great this community is!