They're only trace elements. They're not consumed nearly as much as the big 3 and so take considerably longer to deplete. Maybe with the exception of iodine, which is used in crustacean moulting, water changes and food addition is more than enough to keep the levels of trace elements up.
"Ionic balance" is ambiguous, can refer to many things. A well maintained alkalinity is akin to a good ionic balance as it stabilises pH. I can't be sure the reef buffer products don't contain anything else, but they sure taste a lot like sodium bicarbonate and have the same effect...
The calcium chloride you buy from Kent marine, etc is exactly the same as calcium chloride from a chem supplier.
As for magnesium, some mag suppliments contain a few types of magnesium salts, I'm not sure if one type would suffice, or if you could buy the different types and blend yourself as I've never had to dose mag.