A seller in the FB group I'm in went through something similar until ebay eventually advised them their system is not actually set up to accomodate partnerships so they have remained with PayPal as their payment system - it took a couple of weeks of going in circles, as well. It sounds like yours was handled even worse if they actually switched you over but can't pay out? (I'm not even sure what the issue is, don't they actually have an option to register as a partnership?). 

Unfortunately I don't know what else to advise - you could try raising a complaint against Adyen (the actual payment processor for MP) with AFCA to at least get the funds that are currently being held.