Indeed if an order was posted with 0 quantity ordered, it is considered a complete line and you cannot edit it at a later time to correct it. I believe the last response about suppressing on invoices was simply to ensure that the printed invoice didn't have the zero item and be confusing to the person receiving it.

Editing the quantity ordered or back-ordered of an item to zero and posting will set the status to Complete, regardless if it's the initial posting of the order or an edit of an existing order. Is there a desired way to in some cases allow re-editing and in other cases post as Complete.