The neck piece, ie white, frill and bow are separate to the main dress anyway (see the shadows). I think I would be inclined to sew it all, and edge it in black to give an idea of it being outlined, ie pink frill with edging finished in black overlock/satin stitch, same for bow (edges finished in black), black saddle-type stitching with 6 strand embroidery thread on white collar, button band edged in black piping and then do the buttons the same. I'm sure I have seen on my travels, although not sure where, self cover buttons that have a coverable outer ring and then a separate coverable inner button, but I may of course be entirely making that up!