Neumann’s book on Baroque ornamentation mentions that JS Bach didn’t care whether the ornaments started on or before the beat as long as they made musical sense, but his son, CPE Bach, wrote a book stating that the ornaments should start on the beat. But then, after the book was published and widely read, CPE Bach changed his mind and loosened up. There were so many types of ornaments floating around at this time, and so many ways of playing them, that musical performance resembled jazz in its freedom of expression. Finally, composers got tired of hearing their music so covered in ornaments as to be unrecognizable, so the practice began to be discouraged.
At any rate, I find that listening to recordings of early music specialists is a great help to understanding the style of the period. I recommend Andrew Lawrence King and the Harp Consort.