Thought the "no, you can't have a ship and go out to this classified area you were just at" made little sense, given how important Kirk was to the federation at the time. It was easy to explain their situation to the brass, given McCoy's stuff going on. So the stealing the ship and sabotaging the excelsior portion of this felt a bit cheesy to me.
Of course at the time Kirk asked for the Enterprise he had no idea that Spock's body had regenerated on Genesis. All he actually had to do was bring McCoy to Vulcan so Sarek could get Spock's katra.
Unless of course there was a deleted line where Sarek said "oh, by the way, we need his body too."
Mmm... Sarek did
say "why did you leave him on Genesis?" implying that they shouldn't have left his body there... I might just have invalidated my earlier point...