Interesting theory,but I think fredo really wanted to be in the business, to prove himself to Sonny and Vito.He was most likely always a screw-up due to his weak nature,and he desprately wanted to show he was indeed a Corleone.In the book he's described as a crutch to his father,but never going to really be much in the business. I do agree with half of what you said though,I think Vito would have much rather had Michael in the business than Fredo.