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In The Godfather, Carlo beats Connie, apparently knowing that this would cause her to call Sonny, who would drive over in a rage and have to pass through the toll-booth where the assassins would be waiting to ambush him.

However, Carlo's attack on Connie was a reaction to her going crazy about the dinner she prepared, rather than something premeditated.

So how was the setup supposed to work if she didn't react that way? Were the assassins just waiting on standby until the next time Carlo felt he had the pretext to beat Connie up?

You've mis-remembered. Connie is enraged by a phone call from an anonymous woman, which was most likely part of the plan:

However, the "betrayal" by Carlo is a 'twist' in both the novel and the movie but the novel tells us that the behaviour of Carlo was instrumental in the assassination of Sonny.

But the enemy was making its plans. They too had analyzed the situation and had come to the conclusion that the only way to stave off complete defeat was to kill Sonny Corleone.

This paragraph is immediately followed by the call from an anonymous woman to Connie (Carlo's wife) which sets her off and leads the row between them and, eventually, to Sonny being killed.

While the movie and the book don't spell it out explicitly, the series of events does suggest that "the enemy" expected something like this to happen one way or another. It's even possible that Carlo was in on the plan and would abuse his wife even if she hadn't reacted in that way to the call.

Is the series of events -- Connie gets call, reacts emotionally, gets beaten by her husband, informs Sonny (who was known to have reacted with fury when he'd previously heard about Carlo abusing Connie), Sonny rushes over and even abandons his security detail and thus becomes victim of a trap -- an over-complicated conspiracy of sorts? Possibly. But the sequence also has a deeper meaning:

The eldest son of Don Vito Corleone, Sonny Corleone has a fierce temper and lacks the cool, calculating intelligence of his father. He is a man who lets his passions get the best of him. For example, even though he is married, he has a long, passionate affair with Lucy Mancini. However, Sonny has another side. As a child, he was softhearted. For instance, he once brought home an orphan boy named Tom Hagen, who ended up being raised by the family. Also, Sonny never hit a woman, which could not be said for many of his associates. After the Don is severely wounded, Sonny assumes the role of Don for his family. But because of his temper, Sonny gets the family involved in a long, bloody gang war with rival families. Eventually, Sonny's temper becomes his undoing. When he hears that his sister Connie has been beaten by her husband Carlo, Sonny recklessly drives into New York to go after Carlo without the protection of his bodyguards. As a result, he is shot to death at a toll booth.

In the end, this is a fictional story, and sometimes the "realness" needs to be suspended a bit to make a grander point.