I think the plot holes in GF2 are accidental. And I believe they made it too complicated. Especially Roth's role remains unclear.
In GF1 it's clear: A villain (Sollozzo) appears, and his (drug) interests do conflict with Vito's. The arch villain Barzini uses Sollozzo. A war starts.
In GF2, the arch villain is Roth. But we never really know what he's doing: The drapes, the connections to the Rosatos, his relationship to Pentangeli (aka Clemenza), luring Michael to Cuba, then having Fredo come to Cuba.
Even after having watched the movie, we're in a greater dark than Fredo ever was.
Oh, I almost forgot: I love "The Godfather 2"