What is the molecular mechanism of ammonia's toxicity?
Ammonia is reportedly not toxic in bacteria even at molar conentrations (PMID 16604417).

However, yeast, plants, and animals (presumably, all eukaryotes) are unable to tolerate high intracellular ammonia concentrations.  The best explanation for this phenomena I've seen is a hand-waving assertion that intracellular ammonia taxes overtaxes the cell's ability to maintain membrane gradients (PMID 7764110).  However, that isn't a particularly detailed or satisfying response.

I agree with your assertion that there's a paucity of literature on the subject - we probably don't know the answer yet!