Hi Cutie Pie,
The story of the 'Guitar Man' production number was created in order to give the special a themed story throughout i.e. the rise and rise-again of a 'Guitar Man'. This was to tie-in with the 'Comeback' aspect of the special plus parts of Elvis' own rise to fame and to chronicle the overall creative aspect of Elvis' talent i.e. Singing, dancing, acting.
Elvis looking over his shoulder at the sit-down shows?....well, EP had a lot of distraction there with those pretty girls and they were so excited about seeing their man sing for them after all that time. I think Charlie Hodge referred to a plumber possibly due to the amount of waterworks (tears) that were shed due to the occassion?
Elvis being 'shaky' during "Hound Dog" - I think he was possibly feeling shaky along with sweaty palms and all during every song...any lesser performer would have not attempted to even step on that stage let alone try and sing a song...it would have been a nerve-wrecking experience especially when you have to re-connect with your audience after eight years in the Hollywood wilderness.