Credit Memo Matching

Question regarding credit memo matching to invoice.

If the invoice to which we wish to apply the credit memo is already matched to a PO, do we:

1. match the credit memo to the invoice?

or

2. match the credit memo to the PO?

If the answer is 2. will the credit memo still appear on the credit memo matching report?

2 other questions:

1. what about if the PO has been receipted? Does this change anything? Different process?

2. How to ensure credit memos and itheir matched invoices do not still appear on AP Ageing Report?

Many thanks