It is important to know your kidney status. Excess magnesium needs to be eliminated properly. So this should be tested while you attempt higher doses.

Have you tried the magnesium lotion? Quite a bit gets absorbed this way, and bypasses the diarrhea side effects.
Morton Epsom Lotion... I post about this all the time.

Here is the link to Doctors Best Brain magnesium:
http://www.vitaminshoppe.com/p/docto...s#.UzwEUaL5SyM

Notice they give the elemental value which some manufacturers do not. I like to use their labels as information as to what portion of the total magnesium threonate is actually the magnesium part.

Notice 2 capsules of 2084mg provide 150mg of magnesium.
So your taking 8 of these = 600mg elemental of magnesium.

The RDA for magnesium in adults is about 320mg a day depending on what list you look at. If you are diabetic, or using drugs that block absorption of magnesium (acid blocking drugs) or deplete it from the body like diuretics, and some antibiotics long term use, prednisone, etc...you would need higher daily intake. The general estimate from food for the average person is about 1/2 the RDA.

So you are pretty high at this dose. Not 100% of magnesium is absorbed orally either. So you don't really know what you are getting out of that 600mg elemental at this time.
Do you get loose stools at this dose? That would tell you that some portion of your dose is remaining in the bowel, but there is no certain way to know how much.

Magnesium threonate was at one time highly popular because of claims that this particular chelate would be more likely to get into the brain...thru the blood brain barrier. I have my doubts about this. I know when I apply my Morton's lotion, I have a nice calming effect, so I do it at night to sleep more soundly. It also has reduced my hypertension quite a bit. I therefore think it is far superior to oral forms of magnesium.

So you could try the inexpensive lotion, daily and see what happens either alone or combined with your oral or a partial amount of the oral. I agree, it is expensive to use so many capsules of the threonate.

The advantage to using the lotion also, for EM, is that you can apply it directly to where your redness occurs, and that is possibly a plus in its favor.

Here is a paper about using the more soluble magnesium sulfate, in high dose for EM....
http://www.ncbi.nlm.nih.gov/pubmed/11847944

I suspect that the threonate chelate does not dissolve well enough for the magnesium to be in ionic form. The sulfate is similar to chloride as it is an inorganic form, and the chloride (as SlowMag) has shown in studies to be efficiently absorbed.

Dr. Cohen who has EM uses a serotonin blocking antihistamine to supplement his magnesium therapy... He gets it in Canada, as it is not available here. But there is an antihistamine here, called cyproheptadine which may "supplement" your use of the magnesium. I think getting an rx for a starter amount, may be worth the experiment. You can only go "so high" with magnesium and then it becomes problematic. Some people who use the liquid forms (sulfate or carbonate)... sip it in a few mls all day long to avoid the laxative effect. Like the PubMed article I put up here. This would be very inexpensive to do that way but time consuming instead.

As to your RX question.... the only way I can see getting this on insurance is to have your doctor write the RX for IV infusion using magnesium sulfate as in the study link I provided here.
Instead of injecting it at home, you could then use it orally measuring out the amount you need and sipping it daily. You'd have to figure this out carefully. Magnesium is used IV, for people on chemo. You'd get this at a compounding pharmacy, or a home care provider, and the insurance wouldn't know really what you were using it for. It might even be much cheaper if you bought it outright this way non-sterile at a compounding pharmacy than what you pay for the threonate type from Doctors Best.