Interesting views but I still cannot see any indication that Fredo knew. As P. Lawrence said, why would he even consider staying in the same house as Michael (not to mention the same town) if he thought he'd get whacked.

And I agree to with Apple that the Michael/Fredo hug at mama's funeral meant, to Fredo. that he was forgiven.
Plus, I don't think the Hail Mary thing was necessarily symbolic of anything to come. It seems beleiveable that in that era, that a Sicilian raised boy may have been taught by his papa to say a prayer when fishing.
TIS