Glad to hear you found a way to get the benefits of magnesium oil and Epsom Salts !
Thanks for links to Mark Sircus's website. I've been on the site before, but I read the info on heavy metals more carefully this time. It's interesting that he says that the chemically-synthesized pharmaceutical grade magnesium chloride (hexahydrate) can be 25 times more contaminated (e.g., with lead) than seawater derived magnesium oil, and yet still be "a thousand times better than nothing." This is a valid point.
Dr. Sircus also notes that while there are small amounts of mercury in sea derived magnesium chloride (tested at 0.004 mg/l), the ultrapure Ancient Minerals magnesium chloride has tested as "undetectable" for mercury by instruments sensitive to thresholds as low as 2 parts per billion. While Ancient Minerals may not be completely free of heavy metals, tests that do not detect any mercury (at least above 2 ppb) show that the amount is substantially lower (and perhaps statistically insignificant, at least in comparison to other magnesium oils).
I would attribute this extremely low contamination of Ancient Minerals directly to the protection of a geologically ancient seabed. Think of all the industrial pollution of modern times that has gotten into the oceans (not to mention the lead mining of the Romans during classical times, for example, which remains so detectable in polar ice that the scientists can estimate yearly lead production in the Roman empire, as if counting tree rings). It's at the point where the environment is so laden with heavy metals that I, for one, don't want to add to my personal toxic burden. I would rather go with the purest product, especially since I use it every day.
Even if most people are not sensitive to the relatively small amount of heavy metals in sea derived magnesium oil, I note that even Wikipedia states that there is no safe level of lead, for example. That's amazing to consider since we are all exposed to a certain amount of "background" lead (e.g., residues from sources as varied as old paint, unleaded gasoline, etc.) in soil, water, and household dust. While there is probably no easy way to avoid routine exposures to background levels of toxins like lead, if we have a choice in the matter, then the best choice is to choose avoidance.
Many people take calculated risks when benefits will likely far exceed harm. However, consider that toxins tend to magnify the effects of other toxins. For example, alcohol can magnify the negative effects of heavy metals many times over. And if you think in terms of total toxic burden, and the fact that the liver (and other organs) can only detoxify so many poisons at once, then I do not see why anyone would choose to add to their toxic burden, if they could avoid it.
It is notable that in recent years there has been a dramatic rise in autism among children (now something like 1 in 130 children), and autism is associated with heavy metal exposures (e.g., dental Amalgam in mothers mouths, preservative in vaccines). This is an entire topic by itself, but it shows that small exposures accumulate over time, and ultimately can have major consequences on health (in this case, on the health of future generations).