Fredo Corleone won the 'bloodline' lottery. He was born into a 'royal' mob family. I don't think the son of Vito and the older brother of Michael Corleone cannot be answerable to or take orders from somelike Rocco, al Neri or Frankie 5-Angels.

So I don't think it matters what Fredo's specific job duties, abilities or skills are. Rather who Fredo is related to dictates his position in life; and who fredo should be accountable to, gives him the title as 'Underboss'. Granted, Fredo did a lot of boning , but he never made his bones. This is the basic requirement to work for the family. On thing is for sure, unlike the other Underbosses, Fredo did not have to earn the title/position.

It is common knowledge that Fredo not too fast on the uptake. Therefore, I don't think the Corleones was truly counting on Fredo to learn the casino business. Sonny or Tom may have told Fredo that they need him to learn the business. However, Fredo is a big walking target who is incapable of defending himself. Therefore, NYC during wartime was a dangerous place for Fredo to be.

Looking back, I did not see Fredo Corleone/Moe Greene as a 'learning' mentoring relationship ala Roth/Greene. Even Moe Greene said that 'he took Fredo in' as a favor to the Corleones as if Fredo was a foster child. Its hard to learn anything for a person who bitch slap you public. Therefore, I don't think any real learning of the casino business occured during this relationship.