...Indeed, once again I'd defend the idea that a thousand stardates amount to one year even in TOS, running from the beginning of the Desilu season to the next beginning (rather than from New Year to New Year). That way, both "Charlie X" and "Dagger" have stardates appropriate for winter.
Regarding Nimbus III, the agreement to found Nimbus III might have been made well in advance of "Balance of Terror"; it's just the sort of shady affair reeking of deniability that the three governments would be interested in before
going public with the information that they are on speaking terms. If the governments can't talk directly, why, let's found this secret meeting place, and camouflage it as a neutral colony where no Klingons, humans or Romulans are in evidence - save for the trio of diplomats that secretly meets in the back room of a bar...