This is the issue. It's not water (although that needs fixing, too). it's not sanitation. Your customers simply aren't liquifying the corn. It's why the hydrometer isn't dropping to zero. If you're making a bourbon, if you drop a hydrometer ranged 12-0 Plato in the "finished" fermenter and the hydrometer doesn't sink to the bottom of the fermenter after all those enzymes you added...something is wrong.
The starch in corn/barley/rye is surrounded by a cell wall made up of proteins, lignins, beta glucans etc. in varying amounts. You need to dissolve the cell walls with heat (gelatinization) before you get to the starch. What you're looking at is a lactobacillus pellicle. All that lactobacillus comes in with the malt, which is rife with lactobacillus. The 90F fermentation you're citing eggs it on.
As for the "Super sour, astringent, skunky, medicinal, and also metallic flavor in distillate especially at higher proofs. Sometimes the skunk works it’s way out once we settle into a lower proof, sometimes it stays through the whole run" Here your customer is describing acrolein...also known by industrial vodka producers as "the peppers". Essentially the lactic acid bacteria is metabolizing glycerol in the mash, which yields acrolein in the distillate.
Warning: If the level gets to high...they might have to evacuate the shop. Acrolein is a strong irritant that was actually used as an irritant gas in World War I. So your customers need to fix this, pronto.....and, of course, not distill mash that looks like the one pictured.
Tell them to add their corn at the higher temperatures listed FIRST to liquify the cell walls, and work their way backwards to the lower temperatures and THEN add the enzymes to saccharify the starch. They are mashing backwards, essentially. Their fermentations are filled with starch, which the lactobacillus is more than happy to consume because the yeast can't eat that starch and therefore isn't competing with the lactobacillus...which is why the infection is happening so fast. If they need to know the gelatinization temperatures of corn, rye, barley....Google.
Silk City....good advice, and I like your Bourbon Labels.....