Well, I went ahead and stirred up the eco complete with a full tank of water. I dumped that water and rinsed the gravel twice. The water was much clearer and the smell was down to more of a normal gravel smell. We'll see what happens from here. Interesting thing, my ammonia levels were 0ppm...
I'm pretty sure everything is dead within the gravel. It was left in my partially heated garage for about four days at ~55 degrees F, followed by spending 3 days sealed in 5 gallon buckets ~20 degrees F. At one point, both buckets were completely frozen. I found three onion snails in the substrate, all of which have remained stationary in my other 29 gallon tank for the entire evening. The MTS all look to be dead, as well as the pond snails and some ramshorns. Who knows, maybe they'll all disappear overnight haha. If not, I'll just watch my chem levels and go from there. I know my plants are ready to be out of their cooler though.