Wonderful posts all, guys! Another great exchange on these boards! I'd like to respond. Not disageeing with any of you (nobody's speculation is wrong on this subject), just follow-up thoughts:
DC: It's true that Vito would always be in danger--but then, all Mafia Dons are constantly in danger from other families and from climbers like Sollozzo. Vito's biggest mistake was in underestimating the danger that his "no" to Sollozzo posed to him. Sollozzo might have been a continued danger down the road. But if the newspapers exposed the McCluskey/Sollozzo alliance, the cops would have been forced to hunt down Sollozzo and either jail or deport him. That'd give Vito time to learn from his mistake and take precautions. But even if Sollozzo were killed, he'd still have enemies.

Last Woltz: It's literally true that Michael would never be the same after committing two premeditated murders. I don't think anyone would be the same. But Michael was never publicly identified as a suspect in the S&M murders, and the [SPOILER] Bocchicchio confession cinched it. Of course other Mafiosi knew, and would regard him as having forfeited his "civilian" status. But as a counterpoint, I'd offer that, having killed two guys who weren't members of the Five Families (Sollozzo was essentially a Sicilian freelancer), Michael stood a better chance of not attracting long-term vengeance than if he had. The peace settlement that Vito negotiated effectively saved Michael's life. Barzini moved against him not for killing S&M, but because he saw him as a weak and vulnerable successor to Vito--a job that Michael chose. The key phrase that you used is "The only jobs he could have gotten with enough power to satisfy him would have been connected to the illegitimate world." Exactly! "...to satisfy him..." Had he been satisfied with civilian life, he might have achieved a semblance of normalcy. To echo DC: "Difficult, not impossible."

dt: McCluskey wouldn't be exposed "just for the hell of it." The objective would be to neutralize him. No doubt the "blue wall of silence" would protect him from actual prosecution (though the Corleones' considerable political muscle might result in a case being made against McCluskey). But, after the newspaper stories, I'm reasonably certain that the police brass would move McCluskey to some other assignment, or even suspend him temporarily, to keep the heat off the department until the flap died down. He'd be in no position to help anyone mount another attempt on Vito. And thereafter, McCluskey would be of no use to organized crime--damaged goods.