It all sounds like the work of someone who heard what they were writing. Proof is so many passages not taking the easy route. Makes more sense to me, he wrote away from the instrument and perhaps tried things out later but nobody knows for sure. It was three hundred and fifty years ago. As life moves on you see many habits of convenience but in Bach not a trace. Wonder how quickly or slowly he wrote all that? Plus many children and a job to complete each week for the church, vey zmir.. What I wonder most is did he prefer doing it at the instrument or on paper not necessarily by the clavier.