Probably not as far-fetched as you try to make it. If you read the book, Carlo gradually beat up Connie a little more each time. At first, it was just a black eye and then a few whacks. But the beating that Carlo gave Connie that lead to Sonny's death was severe.

I doubt very much that Barzini's men were waiting for Sonny multiple times. Carlo was most likely told to ensure that the beating would be sufficient enough to incite Sonny into a blind rage that clouded his judgment and forget about his safety as the temporary Don.