Corals do use magnesium I currently dose 210ml of magnesium a day to keep my parameters stable !!! Magnesium is also important to stop precipitation of calcium and alk
out of solution.
The risk of elements that could potentially leach from those rocks seems to much of a risk to take in a marine quarium as it's not like loosing say a £5 plant being a cheap one when a cheap coral will set you back £40.
Copper in a reef aquarium is a big no no it will kill your corals and inverts but is fine for FOWLR
( fish only with live rock )
Myself personally would use the correct rock so you know what's going into your tank I keep a full reef myself so speak from experience