As has come up many times on MPDC, this will require the original owner to contact DJI to resolve the issue. You'll have more success grafting gills onto your throat and breathing underwater than getting DJI to unbind this from the other account...
If you still feel the need to resolve it properly, then I'd recommend contacting DJI support and explain the dilemma, as it appears you still have communication with the original owner who wishes to unbind it, unlike a found drone where DJI won't...
While I agree 100% with everything you said, as an inactive Private Pilot I'm not any more comfortable with the 500m limit than 1000m.
Unless DJI (and other manufacturers) are going to engineer in some sort of reasonably accurate AGL system...