It there anything wrong with your water? High mineral content? Bad ph, is it far off from 7?
If you know for sure that they are drinking plenty, and that your water is free from issues.... then maybe plan for your one and only vet visit.
Make sure you have your list of questions, including what parasites are common in your area, how to tell when your goats have them, and what the local parasites will respond to.... and the amount needed per goat, how to administer... also maybe have them show you how to do a blood draw and how to castrate.....
If the minerals are out 24/7, I am thinking that they do not need the minerals added to the feed. They may be getting too much... like force feeding them....
Your math looks right. I have no idea of the proper balance of minerals for goats.
Walnut has a poisonous nature, in all parts but least amount in the leaves, that I am aware of; however most do not eat them in any quantity ... and they would only do that if they were extremely hungry and that certainly does not seem to be the problem..... but it could be causing some of the discoloration. And throwing off their gut microbe balances.
They don't get perfect in nature... they get what is available seasonally.. I think you are overthinking this and trying too hard to get perfect.... force feeding mineral is like you taking an extra vitamin pill just in case after eating a perfectly balanced meal... and it is going to get pee'd out when your body doesn't need it... to the point of you can actually overdose on some minerals and such.