The white-curtain-on-the-window idea is good. If it's not too thick, it should let enough light pass through. The DIY desk-softbox idea that you suggest would also be fine for the purposes. Probably the best, most simple and most convenient setup. And yeah, the light above should probably be avoided. Wrong colour, and it comes from the wrong place. As for the powder, I think you can safely skip that
WRT framing, I wouldn't worry too much about looking at your notes. It didn't bother me, and I hadn't really noticed. But having a good framing would make a nice improvement. Just make sure you are centered left-right, with your head close to the top. If you do the redesign (not necessarily necessary, imo, if you build your desk softbox), yes, the ceiling can be left white. Regarding the background, I think you can eyeball it, doesn't need to be exactly 18 % gray. You could just take the cheapest gray they have (here, they usually have a rack with discarded, discounted paints cans that didn't come out exactly as intended). They idea is just to have something a bit darker than you so you don't fudge with the background. Ideally wear a shirt that's lighter than the background.
On the whole, I think building that minimal softbox, on top of making good use of the window and choosing a shirt that'll make you separate from the background, would possibly be the Pareto solution. If you repaint anyways, you might as well do as discussed above, but I don't think it's necessary. It looks nicely homey with the painting and all. Depends how much effort you want to put into this, what looks you want, and how good a result you desire.
(I must admit that I've been curious about those hidden books since that interview came out.)