Is he absolutely sure he generated a seed himself? There have been incidents reported where people have received hardware wallets from 3rd party sellers with a 'scratch-to-reveal' seed phrase included in the package and with the seed 'pre-loaded' onto the device.

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?

C++

This could be a really powerful feature for sidechains - the benefits of merge mining but without needing Bitcoin miners to do anything.
But if I understood it right, someone or a group needs to keep paying the Bitcoin to keep it going. Who would pay if the sidechain is decentralised? (like with proof of work or proof of stake)

You might want to look into the privacy angle: i.e. atomic swaps that cannot be linked across chains by anyone other than the parties involved.

This is a good place to start: https://joinmarket.me/blog/blog/the-half-scriptless-swap/