I'm sorry I can't give you any advice above and beyond what you have already accomplished in ATTI mode. Perhaps you can explore a config file, similar to the 32 channel 'hack', that would override the compass error and allow you to fly. Curious to see if we have any talented and willing programmers that would be able to help. It would be great publicity for DJI so I would ask them for a similar config file or customized firmware if you have a reasonable means of communicating with them.
Sorry you are having to deal with this shortcoming but what an amazing experience to add to your resume.
Good luck, keep us posted and stay warm!
Regards,
Albert