While it was theoretically absolutely clear to me what I had to do, it turned out to be more of a brainbendy thing to do than I had expected, and there are a few irregularities where the repeat at the edge is only half there due to the stitch count. Plus I sometimes had to go a few rows down with the decrease stitch, too, in order to make the proper cable crossing.
So the result is not perfect, but it's okay for me, and since this all takes place right at the button band edge (which is looming ever nearer in the future)I figure nobody will notice after blocking and finishing.
So. Next steps: Sleeve knitting, blocking of the fronts, and figuring out the button band details. I'm not sure yet if I'm not going to add an extra button, and whether the buttonholes as described are what I will actually use, so there's a bit of testing in my near future...