Shouldn't this either be getExtendedMasterPrivKey, or allow for an index? e.g., getExtendedMasterPrivKey(i), where i = 0, 1, 2, ... , n. Since there are many private keys. Same thing with the public key below.

Also, it seems your idea of a multisig wallet is different than my own ... yours seems to have the private keys contained inside. I presume only one person's master private key in this wallet, and each person has a different version of the wallet, since other people will have their own master private key inside?