Tim,
Apologies for all the delays. I did a good amount of development using the CB 'recommended' method for creating a user. That turned out to be a rabbit hole for what we need to do though and eventually be an impossible solution. With a lot more investigation into the issue regarding creating the users, we implemented a very simple solution which fixes a common issue that other users have experienced. That issue is that a user can't be registered if there are required fields set within Community Builder that doesn't have any profile mapping setup for it.
I went ahead and implemented the change on your site with the FTP credentials you sent. From our testing, it shouldn't cause any issues. I can't guarantee it will solve your issue, but I think it very well may.
Please test and let us know how it goes. We're planning to test further and this change should be in the next release either way. I'd love to hear your feedback on how it works for you though.
Thanks,
Alex