1. Based upon the location of the searcher, offer distances in KM or miles appropriately
2. Allow the end user to switch between the two.
We are using the plugin internationally and some of our colleagues in Europe do not like that we have it set to miles. So far we have the base plugin and power (but not experience or premier)
If I understand your question correctly regarding the distant unit that you select under the Settings/ search:
Basically what you are asking is for the location sensor aware (available with power and only on secure sites per Google rules ) to also be aware of the locales preference for Kilometers instead of Miles.
There is an option to hide the radius dropdown available with the experience add-on.