Accrual Reconcilation Report and PO difference

We are looking at some old POs  from prior years that are showing up on Accrual Reconciliation Report. Apparently, nobody monitored Purchasing application till recently there was an  audit with a very large variance amount in question.

While reviewing old POs we found following issues:-

1) PO not fully matched to invoice

2) PO over matched with invoice

3) Supplier item was returned after invoice was created. (This is one of our biggest issue). When a supplier item is returned after it was invoice, the item was returned but there was no debit and credit memo was created.

4) Intercompany POs without any invoices or some intercompany with credit memos