The Donship any role how important, nothing but the Donship would have sufficed for Fredo as we saw during Fredo's boathouse outburst Don Fredo [albeit Roth's puppet] The only way this could happen is if the current Don, Michael is dead

Donship is the exact carrot Ola dangled and promised Fredo after fuelling Fredo's resentment "I'm your older brother Mike and I was stepped over!"