My take, for what it is worth!

“something in it for me -- on my own” was, in my view, the Donship and the only way this could happen is if the current Don, Michael was dead or in prison

My take, among others -

• Ola's phone call was a warning, veiled threat “Your brother's not going to find out we talked” Don't go squealing or else....
• Fredo's "You guys lied to me" was because Ola didn't deliver on his [*empty promise carrot] - Michael dead and Fredo the new Don - and now Fredo is in deep...."you got me in deep enough already"
• the astute and meticulous Roth would have instructed Ola, to fuel Fredo's resentment and make sure Fredo listened good and inferred [Ref: *Turnbull] the reward is Donship
• Fredo was Hooked! otherwise Fredo would be stuck running Mickey Mouse nightclubs somewhere!
• [Ref: *Evita] Here's the thing Roth didn't have to make the Donship happen, obviously he can't but that was never his intention

Fredo can handle things He's smart -- not like everyone says -- not dumb, smart! Quick and Clever too including after Ola's phone call
"No doubt he [Fredo] betrayed the family and set his kid brother up to be murdered for the Donship" but should Fredo be sleeping with the fishes....

* "Something in it for me"