Okay, so I checked the potting soil bag and it's Hyponex by Scotts, in a blue bag, not MG like I thought. On the back of the bag it states that it contains sphagnum peat moss and perlite to retain moisture and improve drainage. It also says it feeds plants for up to four months.
As for the added fertilizer, it was well under a full dose. I used a full dose divided among several of my other plants, because I'm cautious about fertilizing too much.
I definitely plan to catch rain water for it in the future, but until then I can check about bottled waters at the store to use, assuming that's okay? I can also go look for a kit to test the pH.
I'll do whatever I have to, to make sure my little tree is okay and will survive this, so I really do appreciate your help.