Re Magnesium: I'm no expert, but the bioavailability of magnesium oxide is very low. It exerts its laxative effect by not being absorbed, staying in the gut, raising the molarity of the gut contents and thus pulling in water. Magnesium, remember, will burn under water, pulling oxygen off of water molecules, your gut is not likely to overcome much of that avidity. I'm trying magnesium glycinate, made by Natrol, to see if it will help with cramping. There are other organic magnesium compounds out there too, the Supplement section of a drug store will provide plenty of reading.
Good luck.