I wonder what would have happened if Spock DIDN'T die- he already put his Katra into McCoy for safe keeping. If he survived the repair job, who would be the real one since McCoy/Spock downloaded the Katra into the blank Genesis Spock and he was restored body and soul.
Was that Katra a complete copy like you back up your C: drive and if it was not needed you simply erase it? If not and it was an actual transference was the repair completed by a soul-less shell Spock?
Katra probably doesn't flee the body until the host is dead. Spock was just telling his Katra where to go when he ceased to be.