So this morning, while waiting my turn at the DMV (as noted in the previous entry), I ripped out the whole thing and cast on a lot fewer. Basically, I measured 15" of what I had (my intended width was 15"-16") and cast on enough for that. And after about the same number of rows my piece is barely 13" wide.
So, now I have two options: continue or not. If I continue, I'll almost certainly have to add an inch or so to either side when I sew the pieces together -- a design feature I'd considered (having those added columns of stitches continue up the front and become the straps) and decided against because I don't really want to be sewing four side seams instead of two.
If I rip out again, I can either split the difference between the two attempts and just hope it works out this time, or say the heck with the ribs (even if it's only k4p1) and just go for stockinette, with a more predictable gauge but more boring execution and less forgiving if the size doesn't quite work out.