I managed to rip out the offending stitching on the twill tape, though the trim itself was damaged and I had to cut a new piece. I briefly forayed into hand stitching the damn thing down, but after two hours of tying to neatly back stitch black thread on black fabric, I admitted defeat and ripped that out as well.
Meanwhile, I had managed to get around the inability to buy $4.80 worth of boning by ordering a $20 gift certificate at the same time. Bones of the proper length were on their way, but I was no closer to knowing what I was actually going to do with them once they got here.
Deciding to do paired boning channels had at least fixed the twill tape issue – it was now juuuust large enough. I finally sat down to do the most meticulous basting stitches I could manage, first basting directly through the V of the twill tape, then basting the outer edges around a piece of boning to make sure it was wide enough to work. At the end, I had three neat lines of stitches and confirmed that both channels were wide enough for boning.
I sewed the seams from the outside so slowly my machine kept buzzing while I almost-but-not-quite pressed down the pedal. I hand cranked the backstitching. Sometimes I stopped every quarter of an inch to adjust my presser foot minutely.
In the end, it worked. Only 3 more to go.
A Twist of Metaphor 