I believe that the plan was for Michael to hide in Sicily from the US police, not from rival Mafiosi. He tells Sr. Vitelli that "I'm an American, hiding from the police in my country." I don't believe that he thought he could keep his presence in Sicily, and his identity, secret from enemies forever. That's why he was holed up in Don Tomassino's villa, and didn't travel without two armed bodyguards.
Yes, he took a risk in his very public marriage. But I think he figured that not even a Mafia rival would kill him on his wedding day. And the betrayal came from within--from Fabrizzio, who knew Michael's true identity long before the wedding.