Eruvin 17: A Permit to Carry on Shabbat

What happens if the terms by which you established your eruv change over that Shabbat? Also: The mishnah that closes the chapter - 4 exemptions for those involved in military encampments, including, of course, eruv. Plus: burial of a soldier in the place where he fell. Is he considered a "met mitzvah"? Or does that particular mitzvah only kick in when there's nobody to do the burial? Certainly, in this case, you don't dig up the seat to bury him there. Do the least amount of damage to the surroundings. And: An exemption from ritual handwashing before food. But ritual handwashing after the meal is... obligatory? What is "melach s'domit" (Sodomite salt) anyway? And what about the exemption from setting up an eruv?!