Hi Aleatoric,
Sorry to learn about this awkwardness. I hope my perspective might be of service to you.
Traditionally, if there is to be split ownership/payment for premastering services, the label would purchase the premaster, and the musician(s) would pay for the premastering session (load in and processing).
Since the label had the invoice, they had clearly been asked to pay for the entire premastering service (premaster, refs, processing, and all). The fact that the musician who originally booked the session neither attended (didn't help with processing choices) nor paid for any of it, means that he really wasn't your client, after all. The defacto client is clearly the label.
I should hesitate from making refs for the musician, since the premaster probably belongs to the label, outright, now. Not only is possession 99% of the Law, they are the ones who paid for it, and the musician _hid_ for 2 months. Not cool. Should at least have given you a head's up on some part of this crazy chain of fulfillment.
Job done. Move on dot org.
Cheers