It is finished!
It looks exactly as fabulous I'd imagined and it fits perfectly. Love it!
The contrast hems are invisible on the outside, the zipper is smooth, the hood is pointy, the sleeves change colour and shape at precisely the right place, the steek hasn't unraveled yet even though I used superwash yarn, it is thin, but warm. I am so very pleased with how it turned out.
I had to rip, unpick, and re-do some things twice to get them be exactly as I'd wanted them to be. Sewing up the hems took ages — black stitches with black yarn were killing my eyes, but the result is so worth it!
Among other things that needed fixing, I performed some sleeve surgery to remove ~10 cm from each sleeve at the elbow. When I made my calculations based on the body I’d knitted so far, I didn’t think they would stretch under their own weight so much. The arms also attached at lower point than I anticipated. All that resulted in sleeves that drooped beyond my fingertips instead of ending at first knuckles as planned.
First I put it on (steek still intact!), then I sort of folded the sleeve up where I wanted the red bit to start, adjusted the length, and marked it. Then I basted in some crochet cotton (mercerized for easier removal) and mattress stitched the two parts together. After that I tried it on again just to make sure I'd not taken off too much and then I just ripped out the extra fabric. The mattress stitch secured the knit stitches, so I don't have to worry about that unraveling.
At first I tried grafting, but that didn't look nice, so I went with mattress stitch instead. (The only difference between these two is that grafting is not pulled taut.) The mattress stitch did create a slight ridge on the inside surface of the sleeve, but that actually works really well. It makes the sleeve fold exactly at the change of colours and I quite like that effect.
I also ripped back 5 cm at the neck and made some decreases to shape the neck hole at the front — it was simply too high otherwise. All that after washing, blocking, attaching the hood and crocheting the steek. I had to take out half the crochet steek and undo the other half to below the 5 cm mark, but it was worth it to achieve a neck hole that is not binding or riding up.
It was a PITA to undo the hood seam, pick up the stitches, reknit (twice! there was a failed attempt at short rows there) the neck, re-attach the hood, and crochet half the steek again. But I knew that if I didn't put in the effort in fixing the neck, I'd never wear it, it was just that uncomfortable.
The last thing I did was to cut the steek, put in the zipper, and sew in the bands to cover the zipper tape and steek folds. Sewing the zipper and the bands took ages, but all those tiny invisible stitches will help secure the steek even more.
I am so pleased with how it turned out that I'm actually itching to start another black sock yarn jacket. Top down, all black, this time. I want to be able to try it on as I knit, so that I won't end up with sleeves fit for an orangutan again.