Thank you so much for taking the time to show this in detail! You made some very fine points about using thicker cord and not needing a crazy tension. I have used upholstery thread in the past which can absolutely be too thin, too strong, and damaging to the aida if pulled too tightly. Will switch to thicker cotton next time.
One question for the next lacing step you haven't shown: when lacing near the corners on the last two edges, do you lace your string through both layers of fabric as it folds and doubles up, or do you just go through the outermost and tuck the excess underneath with your fingers or a bonefolder? Or, does it even matter in your opinion?