Knitted with the three yarns together. Gauge totally off, I used more or less size 2, applying the measurements for size 8.
I did not knit with the Garter Single Row Basketweave stitch (textured stitch of the pattern), but with another one, which, I am sure, has a name, but I don’t know it. It’s worked on 4 rows, two rows of garter stitch, two rows of 1x1 rib, repeat.

Once the body done, I picked up the stitches for the sleeves directly on the armhole and knitted them in the round (obviously).
I managed to make my daughter try it on before making the cowl and was afraid by the way the neckline was stretching. The whole jacket is heavy, it was ugly. Picking up stitches for the collar didn’t change anything about that, so I did a row of crochet along the collar foot, I have absolutely no idea how it’s called. Just poke the crochet through the part you want to reinforce, pull a loop of yarn through, keep it on the crochet, repeat along the way. It worked admirably. The neckline is now firm, with no elasticity at all, exactly what you want for a jacket.

Hermione didn’t want to hear about it until this morning, when I showed her the finished object, with the buttons. Apparently it was all she was waiting for, because she wore it all the day after that. Inside the house.