I don't think Clemenza would have gone over to Barzini's side but Tessio would appeal to Clemenza to 'retire' to Sicily rather than have to go to war with him. Frank Costello was allowed to retire after the unsuccessful attempt on his life. Whether Clemenza would accept this situation is another matter, and then of course one could enter a completely speculative discussion about the power of Pentangelli's regime in a wartime situation.

Al Neri seems to be personaly loyal to Michael. He would have to be eliminated as I don't think he would go to any other family.

A wise don would negotiate with Tom Hagen for his services, even though the other families looked down on him. But again, he was in a sense he was the 'son' most loyal to the tradition of Don Vito and I don't think he could go against the Corleones. So his fate in a post-Corleone family world is speculative. Tessio could take him on his personal attorney perhaps.