HP
Another excellent book, and one that deals exclusively with French helmet plates 1814-1870 is, Plaqaues de Shakos de shapskas, de bonnets a poils by Christian Blondieu. It displays the evolution of the headress plate as worn by all branches, cavalry, infantry, pioneers, schools etc.
I'll try and address your main question as best I can using the aforementioned book. When Napleon III became Emperor in 1852 a new shako plate was introduced to replace the fighting cock previously worn. The eagle was without a crown, faced to the right and had the regiment number or school name or other device on the ball upon which the eagle stood. As I understand this was for all ranks, officers being of a better quality. In the case of National Guard troops the eagle faced left. In 1854 the eagle gained a crown, continued to face to the right up until the end of the rule of Napoleon III, with minor changes to details, size embellishements etc.
According to the Blondieu's description the plain ball illustrated in your second photograph is perhaps a National Guard shako as the eagle faces left. If it faced right it would have been worn by veterans or disciplinary companies in line regiments.
Between the years of 1814-1870 the French army wore an amazing variety of helmet plates with changes occuring on a seemingly yearly basis. They would be a wonderful collection in their own right.
As far as confirming kepi's being exclusively being worn on campaign, that would be difficult to determine because limited photographic evidence exists for the Crimean and Italian Wars. However there was a very fine Crimean era oil painting recently sold by Hermann Historica which depicted the French troops taking a Russian artillery bastion before Sevastopol, all were shown wearing kepis. Aritistic license, maybe, but probably fairly accurate as contemporary accounts written during the Franco-German War tell of the French soldiers dislike for the shako. He much preferred the kepi. Further the pre 1860 shakos just couldn't stand up to the rigors of campaign, the weather, normal wear and tear all took their toll. In the decades prior to the American Civil War our troops wore a nearly identical pattern shako copied from the French. They too were of wool covered cardboard (to use a modern term) but when our troops took the field they wore the much more practical wheel hat, a large, floppy brimmed hat with turn down earflaps.
Again I am not an expert on this subject, these are merely my observations based on good books in my own library. Hope this helps.
Larmo
