As you mention in the intro, ammonia is produced with nitrogen and hydrogen found in the earth's atmosphere- therefore going the round about way to sequester and combine the two materials, especially via mining seems to be an incredibly destructive unsustainable process on the earth in and of itself.

So, I had been wondering, if a process of capturing/separating these gasses, such as in an integrated solar pv system that would also utilize the oxygen separated from water...? But, as you say, other inputs for electricity are cheaper at present...

Another idea, you point out urea and within this equation we only look at ammonia as a base for fertalizer- I was wondering what role using composting material, perhaps instituting a national system for recycling  human and food wastes into the agricultural sector- this may be done somewhat now, though can't recall to what extent.