RBC magnesium is not always an accurate way of measuring mag stores - I went through all of that and it all seems redundant for several reasons. Most people are deficient in it especially for those under any stress. I like mag glycine and mag lysine (Doctors Best High Absorption Magnesium).
Gastrointestinal infections are usually the first place to look for pathogens, and true, as you mention that the gut can be effected by high cortisol (stress) but infections can put high stress on the body. These infections will change body chemistry quite dramatically.
There can be a number of infections that can harbor in the gut, bacteria and yeast but also parasitic infection which is harder to diagnose. Protozoa (parasites) can be problematic.
The lighter colored stools are usually due to bile deficiency. Bile production occurs in the liver/gallbladder. Any stress reduces liver function. Nutrients that work in the liver can include the sulfur containing aminos (taurine, methionine and cysteine), taurine increases bile production. Choline is the b vitamin related to bile production. Milk thistle type of products act on both the liver and gallbladder.
disclaimer: the information provided is educational and should be shared with your healthcare provider.