This ticket does not represent an issue with the plugin and is more of a feature request.
WordPress does not appear to allow the unscheduling of drafts. Once a publish date is set, the only option thereafter is to change the date forward or backward. This prevents visibility and queuing of the draft in the non-scheduled drafts column of the plugin's main calender page.
Granted, this is something that WordPress needs to fix but I thought it might be worth asking if a) you know how to unschedule drafts and b) if you think this is a worthwhile feature to add to future version releases of your plugin.