Throwing darts at a wall:
a) It probably would not take too many dead blackworms or snails, maybe hiding just under the gravel, to spike ammonia.
b) Ammonia at that level retards/kills the bio. I think you have "no hope" but to do more water changes.
c) Amendment to (b): except for perhaps Seachem Stability, if you believe their blurb: "When other bacteria begin to die off (usually from high organic loads caused by the undetected death of an organism), Stability® simply works harder and grows faster!" In my readings, I did see mention of nitrification bacteria that thrive in high ammonia environments (typically used in waste water treatment).
d) I don't think science fully understands this stuff. Only recently did a study come out that found that the nitrogen cycle in freshwater tanks is predominantly driven not by bacteria but "archaea". For our purposes they are still a single-cell organism, but I don't think anybody really knows which organisms end up doing all the work in "mature" aquariums. I wonder if this is why some "delicate" fish do better in tanks that are "well established" -- for the first few months in a tank the bio is, perhaps, still adjusting from organisms that handle high levels of ammonia (early on) to those that out-compete the first wave of bacteria when the ammonia and nitrite levels are consistently low.
e) It is a small tank. Perhaps pull the plants, save some snails (if you like them), and hose out and/or boil the substrate, and begin fresh?