Emmett's guitar playing is completely mis-matched with what is heard on the soundtrack. He plays the wrong chords, doesn't position his hands properly for the chords he does play, and is often strumming when he should be plucking (and vice versa).
During the Hollywood sequence, Emmet's band is backing a black vocalist. In the 1930s, musicians on film were, with extremely rare exceptions (e.g., Benny Goodman's quartet), strictly segregated. There's no way a white band would be playing behind a black woman.
The guitar that Emmett destroys at the end of the movie is (thankfully) not a genuine Selmer Maccaferri. The shattered fragments clearly show a bolt-on neck. The genuine instruments used a bolt during construction, but this bolt was removed before the guitar was completed, leaving an empty hole. (The instrument actually destroyed was a disposable prop created by Michael Dunn.)