I wonder what kind of water Ancient Minerals uses to make the Magnsium Oil? I am assuming that the product comes up as solid salt from the Zechstein sea bed; perhaps I am wrong. Perhaps it comes up as fluid? If not, it would be interesting to know if they are using distilled water or not. Iknow you said that the bath flakes have a high water content, and if they are manufacturing those, I wonder what type of water they use?
Anyhow, I have a question for you: where do you source your magnesium powder? Maybe it's a good option to try.
Alternatively, I may give you suggestion a go: buy the bath flakes and just leave them in a bowl to melt. Do you know about how long it will take for flakes to melt of their own accord?