I have decided to answer this.
You should decide to listen.
The scenario you describe is as follows:
A local collapse (United States centric) of the economic system, coupled with a severe isolationist political leadership which contracts U.S. global military resources.
What happens over the course of a year?
The scenario will not play out as you have outlined. The last action of the first U.S. empire will be an unleashing of the military to usher in the second, and more powerful, supranatural overt empire.
Regardless, to your questions, I can answer to the contingency plan requirements.
1. The most effort from governmental resources will be to preserve the following (in order):
Military command and control.
Political command and control.
Major infrastructure command and control.
Major production command and control.
These are the top four. The largest population centers will be the last given help. Understand that large cities are not production centers, but rather concentrated centers of consumption. The personnel, infrastructure, and assets to provide and organize are of the highest priority.
Food, water, energy, sanitation, and weapons are of primary importance. Those that are not within these basic structures of production will, in effect, become second class citizens. They will be left out.
For the short term, your overall analysis is correct.
After one week of water disruption, the population impact is negligible, as the short term solutions you have listed are perfectly viable.
After one month, the population impact will be negligible in regions with large amounts of freshwater, either frozen or in liquid form. For example, the Great Lakes regions and Alaska. Other areas, which have only become habitable recently in the south and west because of cooling and irrigation technology, will have large dead zones. At least 15% of the population will effectively be deceased or become "water refugees".
After one year, assuming your worst case situation, at least 40% of the population are relocated or deceased. The Chicago area will be the last major city left at this point.
By the third year, the military built nuclear desalination plants necessary will be functioning, rendering the problem solved. A full 60-75% of the population should be eliminated by this point in time, ending in an acceptable loss.
Easily the most overblown factor in the equation as a whole, but not at the individual level. Again, if part of the priority system, not an issue.
For the common, the outlook is difficult, at best. Food, water, and electricity cannot be separated in any way in regards to industrial scale production. As a result, the numbers will exactly match the water projection numbers.
The time-frame is also accurate.
Very salient points, but (to beat the proverbial dead horse), in the end it will not matter. After 3/4 of the population has been eliminated after a solid 3 years have passed, industrial medicines will be plentiful again for the reduced population.
This is all about surplus, nothing more. Currently, within the U.S., there is a surplus of food, power, and water. Remember that the only reason there are "water shortages" is because people do not like to live where it is cold. Not because there is a lack of water.
The surplus is currently tight, but it is there. This means that short term disruptions (weeks) are of little concern for survival, but rather of comfort. This being said, the surplus is not large enough to cushion a long term disruption (years). What this means is simple - the population will naturally drop and relocate until a balance is reached. The population will begin to grow once a surplus begins to be built.