Does the microbiome affect how well you can absorb/use Mg? I've applied the oil, taken mag glycinate, and neither are getting through-- only chelated mag bisglycinate seems to. Will sorting out the microbiome fix this or could it a genetic issue?

Yes, the microbiome has a say in all mineral uptake, including Mg. Mg uptake can be blocked by grains. So if this is your problem, you will want to make sure you’ve eliminated grains. So there can be genetics, microbiome, and dietary factors. Also hydrochloric acid factors in as magnesium chloride is considered to to be easily absorbed. Microbiome improvement should certainly improve Mg uptake which will also improve calcium uptake as Ca is dependent on some Mg for absorption.