Surely it's not going to be in A minor and have a flattened fifth if it is by mozart and it was originally a major key? Possibly it is in C major (yes, yes, same key in a way I know, but probably closer to the truth I would think) and has simply been transposed to work for the guitar better, as often happens. The D# could then be seen as a way of temporarily changin to C minor and the G#, I don't know. I don't really know the piece, but classical music often has accidentals.