Can you get hypocalcemia through your diet?

Yes. Anyone on vegan diet certainly gets much lower calcium than they need unless they receive calcium supplements or calcium-fortified products, e.g., calcium-fortified cereal, soyproducts.
Probably not. Magnesium does bind to calcium but I have not seen hypocalcemia due to ingestion of magnesium at least through the diet. Calcium is regulated by parathyroid hormone and vitamin d. If one has a degree of renal insufficiency they can also have hypocalcemia.