So you have no idea about key C? They don't provide it to you? You don't even know its XPUB? I don't think so, because you wouldn't be able to even see how much money you have, unless they have a specific app that can do it on the background and show you your total holdings.
They provide xpubs and a wallet configuration file that has all the needed data to recreate the multisig wallet. You can't access the unchained seed but you can have them sign txs with it.