Much of what we discuss in this Forum is probably more about FFC's production choices especially regarding II since there is no novel to contribute to it. If Fredo did not bring the $2 million to Cuba, then there's no conversation between Michael and Fredo and, thus, no revelation to the audience that Michael has plans to murder Roth; there's no dramatic revelation to Michael about Fredo's complicity; there's no kiss of death, etc.

That and other scenes leave us puzzled. For example, as cited in this and other threads, why in the world would Michael reveal his Roth murder plans to someone he suspected of being a Roth ally or a Roth marionette? Why indeed! Because the audience needs to know Michael's plan and FFC wants drama and to lay the groundwork for fratricide.