Add-on Restrictive Registrations Fix [Paid]

We hired out a few months ago for an addon that would only allow our Minecraft players to register from in-game. However, in the latest Xenforo update something with registering fields changed and we need the addon fixed. Our in-game API still works for generating and storing a key for the user to signup, but as stated something is messed up on the Xenforo side.

1. When registering with a key you can edit your name
This use to be disabled with a disabled field.

2. When trying to register without editing your name you get presented with the following error:

Due to these errors no one is able to register on our forums. We are willing to pay for this to be fixed.

Signup "redirect" page:

I wasn't directly involved with the creation of the addon and our staff member who was involved has retired and disappeared since. So, I am willing to provide any necessary files, exports, or the such for this to be fixed.