I've never seen (I have looked fairly extensively) any evidence at all substantiating, from any type of measurement, that significant transdermal delivery of magnesium has occurred.
Transdermal delivery of compounds with low lipid solubility is very poor. If we had a drug with the properties of magnesium citrate or magnesium stearate, it would be a major problem in my opinion to deliver even just a few mg of it.
Transdermal delivery can be predicted fairly accurately from physical properties -- lipid solubility, water solubility, and molecular weight. So this is something that can be estimated.
Delivering hundreds of grams of magnesium, and therefore thousands of milligrams of a magnesium compound such as a chelate or the stearate, is not going to happen, from the physical properties of the compounds.
On the loose stools problem, it's true that doses of 250 mg or more (for some individuals, even 200 mg) of many forms are likely to cause loose stools.
The glycinate form definitely seems to cause less problem here. This is why the Elite Pro Minerals is able to provide 400 mg/day of magnesium without dividing the dose, as ordinarily would be done. For example personally I absolutely could not take an amount of magnesium citrate delivering 400 mg of magnesium at one time, but I have no problem with a corresponding amount of magnesium glycinate.
You are more sensitive to this problem than I am, but it could well be the case that an amount of magnesium glycinate delivering 200 mg of magnesium at a time would be fine for you, or almost certainly, for example, 114 mg at a time, 3x/day.
This would work, whereas transdermal would not.