On the first 3 rows you would do the ones at the armhole edge and the one on Row 2 which is the neck edge - purl to last 15 sts, p2tog and work the cable stitches. The p2tog on R 4 is at the neck edge and you keep that there.
BTW, you can do all the decs on the RS row, no need to do purl decs at all. Reverse shaping just means to work the decs at the other edge, it doesn't have to mean you work them on the other side (as in RS/WS).
sue- knitting heretic