Come to think of it what was the sense of Luca's mission anyway? Vito wanted to figure out what was under Sol's fingernails, but all Luca would have learned had he lived was that he was alogned with the Tattaglias, which they Already knew. Barzini never showed his face to Luca. And lets say for argument's sake that they bought Luca's story. what does he do next? Take the money and run bck to Corleone and tell him that Sol and Tattaglia's son gave him 50 K?

At least in III when Michael and Vincent use the same ploy, Vincent learns that Altobello and Luchese are connected, and a plot is set in motion to kill them both.

As for the Roth asassination, it has to be directorial license.
Clearly Roth would have been connected enough to make bail pending trial (or that failing have another of his famous heart attacks and be put in a hospital. Either way that would have given Rocco an escape route.