[sadmon]

Would the Post Office refuse to take it if they can't read the QR code or would they only be bothered about reading the tracking barcode?

They'll scan the barcode.

When it gets to it's destination - if the warehouse covers different companies, is it likely they only have one return centre/inwards goods area (sorry I don't know correct terminology)? And then sort the returns by company?

Every return Is scanned and placed In Its designated area.

At what point would the "delivered" scan be done - by the RM delivery driver or inside the warehouse or both?

In terms of a local delivery, at the collection point, the carrier's depot and when received by the company In the receiving area.

Would the sender be able to be identified purely by the tracking number?

Yes.

The tracking number represents the barcode, which contains the consignor & consignee details. If the barcode cannot be scanned/read, the tracking number Is manually entered and used to Identify the consignment.

Thankyou @mothered