Have you tried painting the base color in a type different from your shadow/highlights?
I've found that using an acryllic for the base color, then shaowing and highlighting in enamel or oils to work best for me... The acryllic isn't attacked by the thinners for the other stuff that way...
Or you could go vice-versa as well...
The main thing to remember is that A: Servicemen are in the sun a lot and squint, so eyes aren't necessarily needing the whites and iris.. B: Unless close-up photography is your GOAL, you needn't paint ALL the shadows and highlights as closely as what's depicted. The closest you can focus on a figure with your eye is about 5" before in goes blurry so for 1/35th scale, you only need include what can be seen on a real person that's standing about 15 feet away from you. C: GIs are going to have a 5 O'Clock shadow, or even a beard, so you paint that part of the face in even fewer shades.. The base color and the hair will be enough.
As for two weeks, I'm still learning after 40 years, lol...