The answer to Q1 may depend on the country in which you are flying. In the US, ATC cannot clear or require a pilot to go >250 KIAS below 10,000' MSL (FAR 91.117.a). ATC is not "the Administrator" in this context -- such waivers must be by formal Letter of Agreement or Waiver, such as those that apply to published Military Training Routes.
This contrasts with FAR 91.117.b, where "ATC" may authorize a speed above 200 KIAS within Class C or D airspace.