It looks like Vincenzo has done most of the job. Coming to your question: "to determine whether anyone actually commissioned this Andante from Clementi as harp music – or is it merely a transcription of an already existing piano work (the sonata cited below)", the text you transcribed gives no answer. The general meaning of it is that there are some clues that Clementi actually wrote for harp, but there is no real evidence about it.

Luigi