Hello,
Many of your points are valid and it seems that it is not possible with standard SAP, but I need to explain why this happens. We are using a tier 1 warehouse management system (in Apparel tier 1 WMS is RedPrairie or Manhattan) that allows for a shipment to be split.
This is not a common practise, however if we are flooded with ecommerce orders and cannot finish all of our store picks (e.g. some are at the loading area where some units remain in the pick process, pack process, PND process etc.) we are able to split the shipment which will allow us to take the units that are in the loading bay and have these send to our stores. The outstanding units will continue to be processed through our WMS and the new shipment is planned and communicated to our stores. When the first shipment is processed through our integration it results in posting the goods issue and closing out the delivery. Having to generate a new delivery to communicate what has already been communicated to our distribution centre does not make logistical sense (i.e. they will not wave this new delivery as it is already in process of being completed). This is actually not a restriction in our WMS software, but is a feature that makes us able to be more flexible.
Thanks for the clarification around standard SAP not allowing a delivery to be partially manifested and processed.