What was Carlo's benefit for the treason?
After Sonny was hit, he lived many more years in the Corleone family. Did the Barzinis give him money? Did they promise him anything? We don't see anything of it. Even more, in the novel we read that when Tom is calling Carlo, Carlo is terrified. And after that he lives the life of a model husband.

I'm sure he probably got some money but the primary benefit to him was revenge for his physical humiliation at Sonny's hands.

I think Carlo saw very early on that he was never going to play a big role in the family business. My guess is he only married Connie for the money and the position, and when he saw heas only going to get a "living" and not be a part of running things, he went back towhatever small time hoods he hung with befoe the marriage and let the word get out to Barzini or Tatt that he would spy on the Corleones for extra chas and some of Tatts women. From there, especially after Sonny beat him up and told him he would kill him if he ever touch onnie again, it was no big deal for Carlo to betray the family and set Sonny up. God knows what Barzini promised him --- it would not have mattered--- had Barzini won the war he would have killed Carlo as sure as he would have killed Tessio.
After the scene where Michael falsely announces Carlo is going to be his top man in Vegas, you can see Carlo smirk as if he has really pulled it all off. You can also see this seemingly bad decision ruffles Tom, Clemenza and Tessio.

I agree with your analysis, DT.
But what did Carlo think would happen after the betrayal? He must have known that if he was found out, he'd be a dead man. And at the time of the betrayal, there was no Michael around.
So did Barzini promise him to take him under his wing? Or what?

Carlo never seemed like a smart guy; he probably figured there was no way he'd ever be found out. I guess he was promised a cut after Barzini took over the Corleones (similar to what was promised to Tessio, I'll wager), but as stated above I guarantee Barzini would have had him killed if he won.

What Lilo said.

Logically, there would have been no way Carlo could ever have expected not to be Suspect Number One in Sonny's murder. But lust for revenge isn't logical. Vito said it all in the novel when he said that "Revenge is a dish best eaten cold." Carlo was probably so driven by getting revenge on Sonny that Barzini could have promised him a candy bar and he'd have done it.

Does anyone know the timeline for how many years between Sonny's and Carlo's deaths?

Sonny's murdered in '46; Carlo in '50 or '51.

I would think that Carlo might have seen something more for himself in the Corleone Family. With Sonny gone, Fredo in Nevada and Michael in Sicily, Carlo may have imagined that Vito, being all out of sons, might come to depend on his Son-in-Law to pick up some of the slack.

Well he prpbably did see something for himself, because Michael played him based on those expectations. He said Carlo was going to be his top man in Vegas after the family made the move. This allowed Carlo to think he had pulled off the set up of Sonny and would rise in the Corleone family. Of course that was a case of Michael "keeping his enemies closer."

This is one of those timeline questions that are obfuscated by sloppy writing and inconsistencies between movies. Carlo was murdered on the day that Michael settled all disputes, aka "The Great Massacre." If you accept the GF timeline, Carlo was whacked with the others in 1955. If you accept the Senate committee chairman's question to Michael in GFII, it's 1950. I believe Sonny was whacked in 1946. See timeline here:

http://www.gangsterbb.net/threads/ubbthr...true#Post580389

Actually it is even more confusing. Whether four or nine years go by between Sonny's murder and Carlo's there's a huge mistake because when Carlo beats Connie to set Sonny up she is pregnant with the child who will become Michael Rizzi, whose Baptism Michael attends. As we know all the heads of the families are killled during the baptism and Carlo is dispatched to the Mall and killed right after it. The baby (who btw was played by Sofia Coppola) was an infant, not a four year old or a ten year old.

Not so fast, dt. In II, Michael chides Connie, "Do you know that your oldest boy, Victor, was arrested for some petty crime you know nothing about?"

Okay, so how old do we make Victor in II? If it is Victor with whom Connie is pregnant in I, then Sonny has to be murdered no later then '46 unless we want to make Victor a gradeschool kid at the time of his arrest.

he would have been about 12, i guess ('46 - '58). And he was just "picked up" for a petty theft which could be anything from shoplifting to grabbing some cash from a street vender. Michael never mentioned being arrested. Just that Victor was "picked up for some petty theft". This all assumes that Carlo knocked up Connie right away- within the first four months of being married as she looked pretty pregnant at the time of her beating.