As far as the 1 million uses for the stuff on the label -- there's a grain of truth for several that I can see. They're not lying so much as exaggerating and using really facile reasoning.
I.e. People are given IV's with magnesium sometimes after a heart attack, because magnesium relaxes heart muscles. That's because magnesium relaxes all muscles. That makes it good (or "good") for just about anything involving muscles: cramps, a stiff back, toss in fibromyalgia because why not, cerebral palsey, and the like.
And, since blood vessels are lined with smooth muscle they'll relax, too and expand, which reduces blood pressure. Reduced blood pressure puts less of a load on the heart. So it's yet again "good for the heart." Since that increases circulation, which would include the brain, then "it's good for dementia" (reality: only if it's due to obstruction of blood flow), and "Alzheimer's" ('cus more blood flow would have to help at least a little, right? -- that's so close to a lie it's ridiculous, unless there's there's some other effect I haven't heard of).
Magnesium is also known to be psychologically relaxing. The proposed mechanism is a mild blocking of NMDA receptors. That's the receptor that ketamine (sometimes used as a general anesthetic) blocks. NMDA antagonists are known to reduce seizure threshold, so it's "good for epilepsy." I wouldn't bet my safety on that alone.
Magnesium is also involved in a various chemical reactions (I think the enzymes that catalyze reactions use it) involved in the production of energy. So, of course, the label says, "good for eNeRgY!!" but that's only likely if you're low on Mg. If you're tired because you have diabetes or anything else other than Mg deficiency then it's not likely to do much.