1. I don't think there's any evidence that Barzini assisted in the attacks on Vito/Luca. Sollozzo tells Tom that the Tattaglias are behind him, but the other families would just go along. At that point, Sollozzo himself should smell a rat if Barzini was assisting but wouldn't reveal himself.

As I've said here before, I think Barzini's role in the initial attacks is simply to tell Sollozzo/Tattaglia that he won't go in on drugs unless Vito is in, too. That puts Sol and Vito on a collision course. It also explains why Sol doesn't claim a Barzini alliance, but Vito is right in saying it's Barzini all along.

2. I think Roth is acting when he brings up Moe. He wants Michael to think that he's conflicted working with Mike, to keep Mike from thinking about his vulnerability in Cuba. Roth also wants Mike to think the suitcase full of money is of secondary importance to Roth.

3. IF Roth actually was motivated by vengeance for Moe, Barzini wins from the grave. Barzini promised support for Moe, which emboldened Moe to dig in his heels, which led to Moe's demise. So if Roth actually was motivated by vengeance for Moe, he was just playing out a hand Barzini had dealt years before.