Issue with Registration codes module in Voucher mode, trying to fix
This is more of an advice than a clean solution to the problem: hack the module and apply the patch if it works for you without interfering with other modules.

As you said, the module isn't maintained well and it is not updated recently (last commit was almost a year ago). If you don't have enough time to go over everything yourself, it is better to just fork it than hope that the maintainer will fix it.

Create a new folder under "sites/all/modules" called "fork" and move the module there. This will make sense later on for you or anybody else working on the site.