I upgraded my Mac wallet and seem to have a new Bitcoin address and private key. Although I used my word seen to restore the old wallet, the old address and private key is not there.
To restore your original multisig wallet, you have to restore it by putting in the xpub (master public keys) of all the co-signers. If you don't, the seed that you used will be considered as a normal address.Create a new wallet by going to File>New/Restore> Multisignature wallet and choose to configuration that you have selected in the first place.
How can I make transactions now from the Linux wallet without the original co-signer Private & Public Master Key/and address?
Restore it using the method above.Can I remove the multisig feature and just go back to a standard wallet. KISS (keep it simple stupid!)?
No. You have to send the Bitcoins out of the multisig wallet to a new wallet without multisig. 