I concur with the notion of Spock feeling he was subject to Federation law for killing Kirk but that he'd be considered exempt from any guilt on Vulcan. There's another issue that has always occurred to me. T'Pau was very well connected with UFP and had even been offerred a seat on the Fed High Council. After the incident, she intercedes on behalf of the Enterprise for having diverted to Vulcan. Surely she would soon learn of the ruse perpetrated by McCoy in feigning Kirk's death. During the rituals on Vulcan, she seemed a real stickler for adhering to tradition and honor. Wouldn't she look unkindly on the deception on the part of Spock's friends? Wouldn't this reflect badly on Spock and hurt his image back in the old hometown?