Good question, Ice--and one that hasn't been asked before. Take a bow!
I'm pretty much with dt here because the basic physics of the situation left no room for maneuver:
--Sollozzo needed Vito dead in order to survive and (perhaps) make a deal on drugs.
--Sonny was adamant that there'd be no deal on drugs as long as the Don lived, and until he could retake the reins of the family.
--Michael wasn't empowered to make a deal with Sollozzo at that meeting. The only purpose was for him to kill Sollozzo and McCluskey.

So, if Michael had lost nerve and didn't shoot, the meeting would have ended the way Sollozzo intended: he was simply buying time to get another shot at Vito.
If Michael did shoot and miss Sollozzo, or just wounded him, and didn't hit McCluskey, he'd be a most wanted man. If McCluskey was quick on the draw, he might have shot Michael. If Michael were quicker, he'd have gone to Sicily. But it might have been impossible for him to come back because McCluskey, to cover his own ass, would have claimed that he was meeting with "informants" (the novel says so), and that one of the "informants" shot the other. McCluskey would have made every effort to identify Michael as the shooter, the better to neutralize the Corleones and any attempt they might make to implicate him in the Sollozzo affair. With the family distracted by defending against McCluskey and the police, the other families would have been tempted to take advantage of the chaos by warring against the Corleones while they were vulnerable.
If Sollozzo was breathing, he and Tattaglia would have continued to try to kill Vito. They would probably succeed eventually. With Vito dead, I'm guessing Sonny's hothead nature would close his mind to any deal with Sollozzo. He'd fight to the last dead Corleone. I believe there'd be a good chance that the family would fall apart, as did the Bonannos after Joe's war with the Commission. Sonny'd never survive--he'd be killed either by someone from the other families, or from within.

As you pointed out, Ice, drugs was a bigger force than Vito Corleone. If, by some chance, Vito survived, I believe he'd make a deal on drugs with the other families, just as he did to get Michael home from Sicily. The condition might have been that Sollozzo had to go, and that some arrangement had to be made to bring Michael home. Maybe they'd arrange a big payoff to get McCluskey to change his mind.