New (to me) computer and firefox is not playing nicely with the web today so couldn't see the whole thing. But...I did just have a conversation with another QB member about this the other day.
When dealing with set-in sleeves...a trick I was taught at a very early age was to sew shoulder seams but NOT side seams of garment. Gather TOP only of sleeve piece and sew into place. Then sew the seam from the hem of the garment all the way down through the seam of the sleeve in one seam. Easy peasy.