An offensive conference is only charged when the offense requests Time for a coach to talk to the batter and/or runner(s). If the defense requests Time, then as long as the offense is ready once the defense is done with its discussion, then there is no charge for whatever the offense does during this time.
Now, that's how it's done in NFHS as you mentioned, as well as virtually every other softball organization I know. However, in NCAA play, runners are not allowed to leave their bases during a players-only defensive conference. If they do, that is an offensive-charged conference.