Yes it seems the upper arm is for adjusting caster. Thinking about the previous design commonly used, when you moved caster it would effectively change the location of both the top and bottom arm position. This new design is more like a 12th scale car, with the lower arm maintaining a fixed position. All mounted on a floating plate though...it will be interesting to see in person.
Regarding the suspension, unlike an electric pan car, you cannot use springs in this class, so the car relies on flex to create the desired tuning characteristics. I have contacted a team driver at WRC in hopes of get a report on the new car, as well as a comparison between the two models. I will share any information I receive on the GT Due thread.