Even though my buttonholes look alright in the end I still think what happened during the process of making them shows why silk buttonhole twist is better to use than poly-cotton thread.
Buttonhole made using double strand of poly-cotton thread. The resulting buttonhole did not completely cover all the fabric. It also was not strong enough to withstand repeated buttoning and unbuttoning.
Since all the buttonholes had already been hand stitched once I discovered how unsatisfactory the double strand of poly-cotton thread was, I had to find a solution. Here it is–I restitched each buttonhole again, using a double strand of poly-cotton thread.
I had to sew another series of buttonhole stitches around each buttonhole. This completely covered the buttonhole so that no fabric showed through. It also reinforced the buttonhole so well that now the buttonholes button and unbutton with ease and there is no fraying.