Clearly not, he lost in the power play. He was attempting to murder Tony but Tony got to him first.

As for captain of Junior's crew, after the War of '99 the crew was disbanded and the territory was collected on for Junior by Bobby until Bobby absorbed the territory or it was taken by other family members.

I see what you're saying, hypothetically, what if.

I don't think Mikey was boss material, all his earner's would have been Tony and Tony's friends. Killing Tony leaves the entire cast wanting to murder you. Tony had New York, Junior did as well but much less so realistically. Junior had connections, Tony had Johnny Sac. Mikey Palmice could have never ran the family with support of Tony's crew or from New York.