Eines nicht sehr fernen Tages werde ich in mein Land zurückkehren.

Is “Eines … fernen Tages” in the genitive because this is an “indefinite” time, like “some day”…whereas if it was a specific day it would be (accusative) “den Tag” or something similar?

Eines fernen (nicht sehr fernen) Tages is a fixed construction, I don’t think there’s much of a complex reasoning for the grammar here.