Regarding brush-painted versus airbrush; The luftwaffe nearly always had all the equipment with them in this stage of the war, and would likely bring airbrushing equipment, and mottle was always by airbrush, even in the field. Your method does look very munch like it was made by airbrush with the blurred edges (and your dots are a lot more realistic than the one I made in this camo with my first try at an airbrush).
Regarding slats these were quite often pushed back in as one of the first actions after landing, and most likely were in for painting. I agree that it is not very often one gets a chance to see a 109 with slats open from above in pictures, but many pictures of them being maintained supports this, and your fine job of painting them RLM02 is quite correct @johnb.
Of course there are some very interesting ones with the mottle done by paintbrush, but I never saw this type being made by paintbrush. (one that springs to mind is the one with dense mottle that is very clearly done by paintbrush being tapped all over the light blue during the early years - one that I really would like to make, but I still have to pursuade myself that it will be hours and hours well spent on the paint) 🙂
