The sweater is garter stitch for about 7", then the sleeves are added by doing four sets of cable cast on (for the front and back). The shoulders and top of the sleeves are 3 needle bind off. But this leaves the underside seam of the sleeves open and they are the bottom horizontal selvedge, not the vertical lines of stitches like you find in Amy's video. So I'm trying to put two cast on edges together. I don't think mattress stitch will work.
I'll play with it a little this pm and see what happens. Thanks!