About the magnesium; been there too: 100-200 mg was maximum I could tolerate in a day. Took me ages, but now I have no problem with the needed amount, ( for me ) approx 600 mg. Also the vit c tolerance has increased last year after I added sulfur.
Take it slow, try different forms of it; I have a hunch that the mag chloride form is easier to assimilate for those with a weak digestion. Good for the bromide too.