re: How much better would Godfather III have been if Duvall was in it?

Yeh, the inner family battle would've been very intense. I don't see as many fans disappointed if that had been the case.
Duvall actually said something like, "You wanna pay Al twice as much as me, that's fine. But five times...come on!" I think I would've said the same seeing as how big of a role he was intended to have. Oh well, at least he got a golden globe for Lonesome Dove by passing up Part III.