If Michael survives the Tahoe shooting but dies in Havana, with Pentangeli in custody, perhaps

Willie Cicci takes over the Corleone operations in New York

If Fredo's betrayal is not found out and Tom opted to 'rescue' the Corleone casino holdings [supposedly legitimate business] and employs Roth's 'strategy' from Roth's birthday party – Neri and Rocco, all of you will share - to make sure things go smooth....

Roth can't finger Fredo nor show his hand, without incriminating Roth in the process

“And Fredo -- well -- Fredo was -- well” –
Big ask to nurture Fredo, giving Fredo at least a glorified face saving role, to show that Fredo is valued in his own right or similar

If Tom somehow found out Fredo's role in Michael's murder, I too “can't see Tom brothering Fredo after knowing what Fredo has done”

I think Tom would probably have taken the vice-presidency of the house and hotels offer and moved his wife, his family, his mistress, them all to Las Vegas even before Michael's funeral!