I still feel there was some sort of hope for Fredo before the Don's assassination. I suppose hope is a very optimistic word when it comes to discussing Fredo, however as we have discussed in this thread, he is present at the Solozzo's meeting even though he says nothing and is present when the war council briefs Vito after he returns home. It's a possibility that in terms of the motive for sending Fredo to Las Vegas is that the family finally realized that Fredo would amount to nothing in his present role.