For #1, the zip code 42223 spans Christian KY and Montgomery TN. The zip code 97635 spans Lake OR and Modoc CA. 95961 can be the city of Arboga, Olivehurst, or Plumas Lake.
The mapping isn't unique. I think the best you can do is restrict your pulldown to the possible choices. Even then, sometimes there are small cities and towns that you don't expect.