For many years automatic PO creation is already ME59N. And it should only be used if your master data ( vendor, material, info record) is in a very good shape so that the users do not need to enter or change anything, e.g. could just click save in ME21N
your second scenario is very vague, your customers order the finished material from the trading plant. the trading plant orders at the manufacturing plant. The manufacturing plant orders raw materials from the vendor to manufacture the finished material.
Why does the trading plant get 6000 if 8000 are ordered, who's fault is this?
What is the reason for an extra Excel sheet (this is usually a fall back if users are not trained enough and do not understand the processes and do not know how to use SAP to track)
if you want automate anything between manufacturing plant and vendor, then you need to tell what you want automate. In general an automation should be done for processes that work well and do not need any human interaction. And automation cannot solve your problems with getting/receiving less than ordered