You are being told something quite different to other sellers in similar circumstances - it's true that for whatever reason, eBay don't seem equipped to deal with businesses that are trusts or partnerships, despite having an option to register for MP under these structures (seriously, how hard would be it be disable this option and include some info somewhere on what to do if you're being asked to register?), but other sellers have just been kept on with PayPal or - if they were registered with MP - meant to be switched back, so it seems they are handling your case differently. 

The other weird thing is that of those that were switched over to MP, payouts were held from the get-go, but it soulds like you've been on MP for a while without issue until now? 

I see you still have listings up - I take it by "parting ways" eBay are leaving it up to you to... just close up shop? Either that or it's a system generated message that doesn't bear any relevance. In any case, if no one can buy anything successfully at the moment, it may be better to put the store in time away until it gets sorted.

I'd still try AFCA (re: Adyen) for the outstanding funds, as well.