Yeah, not sure if retarder or leveler is a solution for enamels - these products were developed for acrylics, which dry and cure much faster than enamels.
I think this is a thinning, standoff, or possibly air pressure problem. The sprayable paint should be thin (like fat free milk). After that, experiment with the standoff distance and air pressure to make sure that the paint goes on wet. I generally keep a scrap wing to experiment on before trying "for real" on a project.
One other possible culprit could be the thinner, and its compatibility with the paint. You mentioned adding Gunze leveling thinner: I don't think it's compatible with enamel-based paints.
Regarding pre-shading: you can always redo it once you fix the surface and apply the new paint.
Anyway, sorry to flood you with commentary. I am sure you will figure it out. I found that "sleeping on it" is the best way to figure out and overcome these setbacks. Keep us posted
18. October at 02:50:24