When Michael is fleeing Cuba, he sees his frightened brother Fredo also running around. Michael tries to get Fredo to come with him, but Fredo, still smarting from the Sicilian Kiss of Death (SKOD), doesn't come because he's afraid of Mike.

So Michael calls to him, "Fredo, you're still my brother!"

If Fredo had gotten on the plane with Mike, and then and apologized, could he have saved his skin?

Put another way, was there anything Fredo could have done after the SKOD to have actually avoided death? Or was it already too late?

I feel like Fredo screwed himself with his defiant "I was stepped over" screed. If he had groveled an apology to Mike, do you think Mike would have forgiven him?

And if Fredo's fate was already sealed, then why did Mike even bother to say "Fredo you're still my brother"? Was he being sincere, or did he just want to make sure Fredo did not get away? You know, keep your enemies closer?

Michael wanted Fredo to get in the car with him (and dispatched Tom to find Fredo after Michael returned to Tahoe) to pump him for more info that was relevant to the Tahoe attack and Roth's plan to have Michael killed. Having come so close to death, Michael couldn't afford to let any opportunity go by to find out more about any remaining threats to his life. This became even more critical after he got back to the US, and Tom told him that Roth survived and got out of Cuba.
Would Michael have spared Fredo if Fredo hadn't made that "I was stepped over" speech? Probably not. Michael had a policy of not giving any enemy a pass. The penultimate page of the novel explains it:
After the Great Massacre of 1955, Kay leaves Michael and takes the kids to New Hampshire. Tom goes there to convince her to return. She demands to know why Michael couldn't have forgiven Carlo and Tessio for having betrayed him. Tom replies that they had to be killed because "treachery can't be forgiven....they would always be dangerous....He would be shirking his duty to you and his children, to the whole family, to me and my family, if he let Tessio and Carlo go free. They would have been a danger to us all, all of our lives."

Well, I feel if Fredo hadn't given that little rant, Michael might have spared his life, but exile him or something. If Fredo had gotten on the plane, confessed everything to Michael (i.e. without Michael having to probe him for the info), and make it seem like he was the victim, Michael might have shown him some sort of pity.

I think after debriefing him, Michael would have thrown him out of the plane telling his mother Fredo got killed in Cuba!!
This is just a joke, since I had nothing to do I post it here to raise my number of posts!!

Another interesting theory is what would have happened if Fredo had confessed at the cafe...

I think Fredo would have had a chance if he had gone on the plane with Michael. When he made a break for it, it showed Michael that Fredo wouldn't trust him even when his own life was at stake.

I think Fredo was 98% done for at that point, and the "I was stepped over" speech sealed the deal.

Fredo was done with since the SUPERMAN scene, sayin that Johnny Ola brought him there after saying he didnt know him. If he would have confessed when earlier he probably would have saved him. After the Kiss of Death scene i believe Michael would have killed him no matter what happened. If it seemed that Michael was being nice its only because he wanted info from Fredo. Maybe if Fredo would have betrayed him in the GF1 or GF3 he wouldnt have been killed. But Michael's personality in GF2 was monstrous.

Well you can see Michael was a bit shocked when he heard Fredo "confess" he already knew Johny Ola and co. so I think he might could've spared him, untill the "I was stepped over" speech from Freddie.