Would having a CWS turret (or more than one) on a buoy be effective supporting regular mines in orbital missions?
No. Regardless of what you mean by "buoy" (missile or fighter or something else), a CIWS unit can only fire on missiles, and only in defense of itself. If the buoy does not also include offensive weapons, there is not reason to fire on it (and at minefield ranges, very little reason to fire
missiles at it instead of beam weapons).
The only way it would work is if your "buoy" was a full-fledged structural shell, ship, or fighter (with or without engines).