Are you familiar with the fact that Epsom Salts (Magnesium Sulfate)are absorbed through the skin? Magnesium oil is Magnesium chloride. One of the substances that is highly concentrated in the Dead Sea...yes, the body of water that people come from all around the world to soak in for it's healing properties.
The concentration of Magnesium in the Dead Sea is 40,000 mg/l. The Physics clearly tells us that the driving force behind transdermal intake is the concentration gradient. The concentration of magnesium in the pure magnesium oil is about 80,000 mg/l and when you apply that directly on the skin, intake rate is very high.
Using logic, lets revisit just a few things that are absorbed through the skin. Nitroglycerin, nicotine, many vitamins A, E, C, CoQ10, Alpha lipoic acid,NSAIDs. It is not the impenetrable barrier that was previously thought.
We use estrogen and other hormone patches, topical iodine, Antibiotics , antifungals, anesthetics, muscle relaxants etc...Anything that is lipophilic (fat loving) including pesticides and even mercury is well absorbed through the dermal layers. I am not certain why you are so resistant to the idea that Magnesium Chloride is absorbed through the skin. It is more easily retained than Magnesium Sulphate which is more readily excreted by the kidneys.
Here is a site that helps explain transdermal delivery..