The thought of finishing the jersey with a crappy placket nettled me all day. It took all evening, but I think I'm on the right track now. I measured my cutting template for the placket and found it was 1/4" too narrow, so I made a new cutting template, I then cut a new placket sandwiched with fusible interfacing for precision. Carefully I ironed the interfacing to the spandex.. did I mention spandex really doesn't like fusible interfacing? Spandex doesn't like to be folded and creased with heat more than once too.
Instead of using a triple straight stretch stitch on my Pfaff (say that three times quickly), I used my Singer 20U with a tight narrow zig-zag stitch, almost an embroidery stitch. I did a great deal of testing to adjust the tension, and it appears that the placket lays very flat without any encouragement to stay in place. So far there is almost no puckering in the spandex as you can see in my side-by-side. The placket to the right is stitched in place and the front has been slashed. So far so good.