Why is protein content in wheat important?  At what stage in the food chain is a low wheat protein content problematic.  That is, when do people notice?

Also, the pre-nitrogen fertilizer techniques you describe, like fallow fields and whatnot - are those from American farming, or European?  Americans had a pretty awful reputation as soil killers in the nineteenth centuries, and hardly bothered with any techniques to manage the soil or improve yield.  There are some interesting passages from the 1860 census on this matter, basically to the point that it wasn't really worth it for most American farmers to bother with any sort of soil management or preservation because there were too few farmers, too few capital, and too much difficult bringing their products to market for it to be worth the extra investment.

There would be a HUGE difference between reverting to old-style American practice, and to intensive cultivation practices found in 19th century Britain, or for that matter China or Japan.

I'm not saying that intensive organic cultivation is the answer or anything.  I have no clue.  It's just something to address, since you're bringing up pre-nitrogen practice as a major point.

Speaking of that, a graph plotting the introduction of nitrogen fertilizers, guano and whatnot, with crop yield growth and population growth might be interesting.  Probably a ton of work to put together, but interesting nonetheless.

On the subject of old-school fertilizers, I know in Edo-period (1600-1868) Japan they used fish-meal, ground up cold-water fish, as a fertilizer for most commercial agriculture.  The demand for fertilizer was one of the factors that drove the development of the Hokkaido fisheries.