I can only offer my best guesses, but, since that rarely stops me....
Imagine such an event, upon a real battlefield, where real generals move their troops. It is a total absurdity. Even if a few scouts fell into the canal, the regular forces would never end up in such a predicament.
Couple possibilities....
1. Spalding was a war veteran... perhaps he experienced some serious battles in which he saw some pretty traumatic things. His attempts at popularizing bloodless wars and absurd strategies (that nevertheless work) is perhaps his way of making sense of war or giving it meaning--either that or simply taming it down for public consumption.
I tend to think, rather, something along the lines of this...
2. Just go to the movies and you'll see any number of totally absurd scenarios playing themselves out. Spiderman, Batman, Mission Impossible, etc, etc--the heroes in these movies--even in the ones like Mission Impossible where the hero is supposedly a mere mortal--repeatedly pull off impossible stunts. Nearly everyone above Jr High school age knows these types of things just don't happen in real life, but they still pay to go see these movies. So these may be Spalding's 19th century attempts at creating larger than life characters.
As to why the repetition of themes... I'm stumped on that other than perhaps Spalding just couldn't come up with new ideas so he merely redressed old ones. We know that toward the end he was writing with a bit more desperation.... his financial situation was closing in on him and he needed to finish his novel and get is published, so perhaps he simply recycled old themes to fill pages.
Do you see what I'm driving at?
Ummm... yeah, you want me to read Spalding's mind. Now where did I leave my seer stone?