Updates

Made a few cosmetic changes, and also one important note ... they would not share the master public key. They would share the purpose key. That is because the purpose key cannot be derived from the master key (it is hardened and thus requires the master private key, which is not signed).

Also I think we ought to consider changing the scheme to this:

m / purpose' / wallet_id' / cosigner_id / change / address_index

Suppose someone wanted to use the same master private key on many different multisig wallets. Without a wallet_id, they would only be able to use one such wallet. But with a wallet_id, they can use a new wallet_id for every new wallet. Thus they can participate infinitely many multisig wallets (or nearly so) with the same master key.

So far I have only read the first half or so and these changes are for the first half only.