1 - you can only delete a PO when the quantity received is equal to the quantity invoiced.
2 - from the posted receipt, from the functions button: Undo Receipt. I don’t know if that also updates the original PO and you can delete it from there, I’d have to try that out
3 - create a purchase return receipt, and post it. When you create the return order, you can use the ‘Copy Document’ functionality, which allows you to get the same information in there from a posted document.