Surely if it's something like 10-of-15, there are still 5 other possible signatories?
So you all have your private keys - I don't understand how it's not possible for you to generate the signatures and the signature algorithm in BCH is unchanged. So I guess this must be just an implementation issue with Copay?
Well now it's 3-of-6. BitPay (developer of Copay) says they need every single signatory's back-up to fix the problem. All of the signatories did sign to create the BCH wallet, so surely it should have been set up, but it wasn't. I guess this would be classified as an implementation issue, yes.