Now that I have done it this way I think I can pick up and knit a half hexagon from these triangles. Or at least I will try.
Essentially, the back is straight across at the neckline and in order to meet it I added a tiny triangle atop the last half diamond (the triangle knit after the last diamond). The tiny triangle is decreased on each edge, like the very bottom triangles at the hem.
Here's a view of the top of the neck, so it is obvious how wide the back neck became, the way I ended it. I am going to try knitting the fronts and back together with a three sided unit, but first I will make a swatch and figure it out stitchwise first. I don't like seams, as they are lumpy.
Here's a diagram of the last unit and the decreases. Those interior decreases will be Sl 1, k2tog, psso. The edge decreases will be just knit 2 tog. I think...haven't done it yet.