The song makes me feel upbeat and happy. I think the mood and what the musician is trying to express is that no one should be drafted to go to The Vietnam War. All of the problems he says he has with his body contributes to the mood.
The audience includes all other men who could be drafted to go to the war, because this song is about a man making excuses about why he shouldn't go to the war.
The part of the song that is fact is the part about men being drafted for the war. The conjecture part is all of the problems with his body that he says he has.