Could Zack have been saved

People often question what would have happened to Zack if he didn't take Cloud with him as well. Assuming that Cloud had died or they were separated (since I really can't see Zack abandoning Cloud otherwise) then I think Zack would have had a better chance of surviving, but I don't think his safety would have been certain. There is still the chance that Zack would have been caught or cornered by a large number of Shinra forces in which case he still would have been killed. The only difference Cloud made was in slowing Zack down.

Let's say Zack didn't have Cloud with him. If that were the case it would be possible, if not probable that he could have taken another route towards Midgar. He would have also been able to run if he had Shinra's infantry chasing him. That being said, Shinra would have continued to follow Zack even without Cloud since Zack knew all of their secrets. So, assuming he made it to Midgar Zack wouldn't have been out of Shinra's reach. If they couldn't kill him they would have had the TURKS monitor him until he died. Therefore, unless Zack was able to fake his own death it is unlikely that Shinra would ever stop chasing him.

Let's go a step further and say Zack was able to fake his own death. How would he hide? Shinra had eyes everywhere and it wasn't like he was a low ranking SOLDIER. Most high ranking members of Shinra would know his face, so he couldn't hide in any small towns or villages where he could easily be spotted. That would leave Midgar or Junon, both of which were epicenters of Shinra's military activity. Since most of the high ranking members of Shinra knew him, he wouldn't be able to hide where they could easily spot him. That would leave the slums of Midgar as the only logical place he could even try to hide, and in fact there was a rumor about just that, how a half mad SOLDIER was living in the slums.