For Kirk, I could see him accepting the promotion, if the Five Year Mission took it's toll on him. Remember, Decker does point out that when he took over, Kirk wanted a command again someday. I could see Kirk accepting it as his next logical course, and then regretting being stuck there. He was probably convinced he was the right person for the job given his experience.
Spock was a singular mind on a world of billions that were distinctly different. I think it was *Spock* not V'ger that did the sensing. As is said during the ceremony "it touches(calls to?) your human half, Spock."