Oh, I have shed plenty of tears over buttonholes gone wrong too, so I feel your pain! When those automatic buttonholes work, they’re amazing, but when they go wrong, they go horribly wrong! And it’s true that making a sample doesn’t always mean you’ll get a good result. It’s taken some time, but I’ve learned how mine works and I am now able to help (or force/pull/push) it along when it gets stuck. I have several tricks, some of which are too late for you.
I agree that grading seams is important. In fact, I never trim seams, especially corners. I grade. Trimming just shortens the bulk, it doesn’t reduce it. Grading creates a gradual transition.
Can you adjust the pressure on your presser foot? When I do thicker fabrics, there’s a dial on my machine that reduces the presser foot pressure and lets bulkier areas through. This is especially helpful with buttonholes.
When the buttonhole is still not perfect, I go over it after I have opened it up with a zig zag stitch. Just adjust the zigzag to be close together and the same width as the buttonhole, but take the zag over the edge opening. This actually makes a nicely bound edge and I’m considering practicing this to do for real next time I have to do like coat buttonholes.
Finally, you mention that you made this big? Why not cut the ruined buttonhole placket off altogether and put in a zipper. The many seams will make invisible tricky because of bulk and because it will be tough to line them up. But, if you use a regular zipper and set it by hand, you can get a perfect line up and lots of strength. And one wrong hand stitch will not ruin a garment.
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