Deleting PM's
Could I ask that you don't allow a Bully's Charter to happen as it will be well and truly abused.
I would be quite happy for someone to delete a sent PM, providing it has not been read by the recipient. However, as soon as it has been read (blue ticks), nobody other than the recipient should have access to the recipient copy.
Otherwise evidence of bullying can be destroyed by the bully and would only leave screenshots to be proof of receipt.