Anthony, You never said exactly what forms of nitrogen (N) are in the MG shake and feed you used. Neither urea nor ammonium (NH4+) salts release ammonia upon solubilization in water under typical circumstances. In water, urea is hydrolyzed to NH4+. NH4+ is converted to NO3- in a 2 step process in which each step is mediated by a highly specific group of bacteria. It is well accepted that the nitrifying bacteria exist in isolated colonies rather than being homogeneously distributed through a potting soil. So, the heterogeneity in pH & other chemical features driven by the nitrification process may be very large, particularly if your media has a low-buffering capacity as is common in potting soils. The fact that you detected significant ammonia oder indicates that any nitrification process was going very slowly in your media, the ammonia was volatilizing leaving the H+ in solution to drive a fatally low pH in your rhizosphere. Other toxicities associated with this process almost certainly also came into play. When growing in soilless media, you need to be very careful about the NH4+/NO3- ratio of any fertilizer you use. You want to keep the amount of NH4+ much lower than NO3- in most cases. Slow release fertilizers may not cause problems if the slow release of urea and/or NH4+ do not overwhelm the capacity of the bacteria in your media to convert to NO3- without using up the buffering capacity of your media. That said, I personally would avoid even slow release products with high NH4+/NO3- ratios.
By the way, I would toss that old soil. It is chemically/biochemically a big mess. I am happy to point anyone that is interested to the scientific publications that support what I said above.
Good luck with the next cuttings!