A MAJOR PLOT HOLE(Spoiler Warning)
Okay,
Maybe I'm missing something here but why did Kirk and crew need to steal the Enterprise in the first place? When Sarek visited Kirk and mind melded, the concern was more for separating Spock Katra from McCoy's mind by taking him to Mount Saliya to be interred at the "Vulcan Hall of Ancient Thought". While the Grissom had discovered that Spock's body had survived the landing on Genesis, there was nothing in the film indicating that Kirk had known about this no did he know about the ritual(Fal-Tor-Pan?) which was what put Spock's Katra back in his regenerated body.
Kirk should have booked passage on the next civilian ship to Vulcan and taken McCoy there,