Can I confirm: if I create a 2 of 3 multi-sig paper storage wallet, do I need the Master Public Key of all three accounts to access the funds again? Or can I do it with just 2 of the 3 seeds?
For each cosigner you need either the seed or the MPK. So 1 seed + 2MPKs, or 2 seeds +1mpk, 3seeds+0mpks, or 0 seeds +3 mpks. To sign a transaction you will need at least 2 seeds in that wallet or you will need to carry the unsigned tx to a wallet that has the necessary number of seeds. You can't sign with the MPK but you need it to reconstruct the wallet.