The main problem with too much magnesium is it can overwhelm the kidneys, so patients with kidney failure should not take supplemental magnesium. Usually in a person with normal kidney function 500 - 800 mg/d is safe. If he was taking it not to fix constipation but to induce weight loss by abusing it as a laxative, that would fit into the whole eating disorder thought.
And frankly, I'm not surprised that you think your H has NPD, because the weird way he writes totally sounds like a crazy narcissist to me. (As for your brother, who knows if that diagnosis was correct, I think teenagers in crisis and using drugs get mislabeled a lot. Does he strike you as a narcissist now?)
It does sound like the family dysfunction in your childhood may be the reason why you never recognized it in your H - you were accustomed to it. It felt familiar.
(In my case, the opposite was true - my ex was a fairly benign narcissist, and nobody in my family had any kind of pathology like that, so I feel like I was naive and just didn't know what to look for because I'd not been around it.)
