Stop dosing. Keep your carbon and Phosguard going, and let the tank settle. Every time you do a water change, you hit the 'reset' button on the tank finding its own equilibrium.
What salt mix are you using? If you only dosed MG once, you should not have had that crazy high level.
I was going to question the ammonia reading but if you put in a rock that was out of the water for a day and had Zoas and stuff on it, there may have been some die-off. If you are using an API test kit, I'd still question that ammonia reading, but I'd recommend a dose of Prime, just in case - if it is positive, Prime will bind ammonia (and nitrite that will come next) and render them less toxic.
You are definitely over-stocked, and the heavy feeding that carnivores need, will lend to the phosphate issue, but that's the least of your problems at the moment.
I'd suggest rehoming some fish, keep an eye on ammonia, nitrite and nitrate as well as pH in the short term, don't tinker with it and let it settle. The more you mess with CA, Alk, MG, the more it's going to teeter totter. Let it be. Once it has settled out (about 3 weeks), then if you need to dial it in a bit, you can do that carefully and gradually.
Jenn