Jon;
NICE start! I have this kit and it's always pretty informative and very interesting to see how folks do it up.
Firstly, I know nada about the "mapping" or much of the other formal approaches and "schools" either.... so cannot comment to nor endorse, or not, any such! I do go with the pre-shading and some panel-fading, etc., and washes are a way to go (IMO).
Couple of ideas:
From where I sit, I'd be thinking of adding more very thin van dycke brown oil wash in the shaded areas to pick up some additional contrast. The zimm and the fender texture would, to me, pop out a little more with more wash down into the holes. (And... When you apply your white wash over, keep that light and don't fill into those zimm holes - which would not likely have been filled in with whitewash on the real thing anyway - unless the crew was really really liberal with that wash and dedicated to doing so!).
I'm not sure where this might fit into your scheme, but maybe do some wear and fading on the yellow coat to age it - maybe some chipping and wear-down on edges and corners before going into the white? This could capture a seasoned vehicle getting a white temp coat for the winter - naturally, the gelb coat would be the older, so it should be a little worn. The fender surfaces could gain, I think, from some dry-brushing with dark grey or a steel colour to hi-lite the diamond plate pattern where paint would naturally and quickly wear away.
I'd skip the hair-spray on this vehicle _ I don't think you'll gain much desired variation in your whitewash coat over the zimm - now, if you were going onto a typical smoother surface, yes... Here, I think you'll get good effects by either careful spraying modualtion, or by actually brushing on some thin white (this could do what you want if you are in fact thinking of emulating a brushed-on or broomed-on finish, as versus a sprayed - on one).
Do some more pinwash and use something like well-thinned burnt umber.
Depending on how used you want it to appear... maybe dirt and scuz-up those tools some. Yours look really good so far as I can tell, but maybe consistent and newish in appearance? Shovels, etc., get used and the blades could likely be variably mudded and maybe even partially rusted (hey, my garden shovel does that!). Your jack block looks good, too - And there's been some interesting thread elsewhere these - I like the wood look. As this one is large and in the open, it could gain some by some mudding details to make it pop up a bit?
For numbers and balkankreuz.... I almost always spray these with stencils. I have never had good luck with decals on anything like zimm, and found the spraying-on to give nice realistic representation of numbers which were sprayed on... Of course, that's me and I happen to have some stencils, so...!
Of course, these are just my opines and suggestions - I am hardly the expert on either the Stuh 42 or modeling!
Overall, I really like it! Way to go and I'll be watching for your finished posting! I am sure it will be quite nice however you go!
PS: On blackening Fruils.... I encountered that same issue with a set for a Hotchkiss 39 - I had placed a complete run into a jar with Blacken-It and let it stand around for awhile. The colour was amazing! And I did get those "silvery" holidays where links touched and blackener did not get in.
The easy solution - worked out for the next run - turned out to be simply sloshing it around several times during "steeping", making sure to move links and flex the run a bit. Also, pretty much like the acceleration in passive chemical plating obtained by agitating the plating solution, the blackening of the second run went much faster! And no holidays, either!
But your tracks sure came out looking pretty good, however you handled your gaps in plating!
Bob