Going backwards is NOT advisable, particularly since password formats were changed between versions and so any new customers added, and anyone who changed passwords, won't be able to log in.

A faster fix would be to either apply the updates mentioned in the DIY thread, or switch to the built-in Responsive Classic temporarily.

And, all this could have been avoided if the upgrade had been staged in a separate copy of the site first, as advised in all the readme docs, upgrade instructions, etc. (I'm not making this comment to point fingers or such. Just drawing attention to the fact so it can be noted for future, or for others who read this discussion.)