And this is what it looks like today:
And the other side:
We decided to have the hired help do this work since I'm unable to haul heavy OSB upstairs, and my ability to climb into the attic is diminishing daily. While they were measuring and marking and cutting and attaching the studs, I worked on hanging insulation and attaching metal plates between the new studs and the old roof framing.
The dark wooden post in the last picture could not be removed, so we had to angle the wall to avoid it. (If we had been able to take it out, we would have built a bench seat over the ductwork.) From there, it goes straight back to meet the other wall at 90 degrees. It'll be a trick to frame that angled portion (attaching it to the roof framing, specifically), but fortunately it's not going to be my brain cells grinding together to figure it out.