Priority Location

I have two branches (Each with Warehouse) and one Head Office (Main Warehouse)

Shop A has purchasing powers and local state taxation with Warehouse
Shop B is within the same state and just has a warehouse - distributing facility. So when a sales order for Shop B is processed, priority of Delivery Order should be to pick location of Shop B and then later from Head Office.

However if the order at Head Office/Main Shop is processed it should not pick-up Shop B stock as first, but only if stock is not there in Head Office Warehouse. I thought Pull flow would be the solution for my Shop B, but found out I have to assign it to each product....

Update of Delivery Order Assigned Location

Can the assigned location for goods be changed in Delivery Order Automatically? Because if my HO shows Shop B location in Delivery Order, but it is more cost effective for me to buy the product from my supplier rather than ship from my Shop B, So can the delivery order update it self later that material is now available from local location purchased from Supplier.