"Now, the voter makes his vote. He generate a nonce (unique number used once), makes his vote, signs it with his keypair, and encrypts this with the public SMPC key. ... Since the vote wasn't encrypted with the voter's key, he can't decrypt it which means that nobody can prove what he voted for using the encrypted message."

Eh, he didn't encrypt it, but he signed it. How can people not prove that he created it? Whoever holds the SMPC key knows everybody's votes.