It's not clear at all to me. C'mon, get real, you make it sound like a legal document ! Why would anyone want to falsify a PM ? Is this something on which you may later rely on in court ? I really don't think so.
Ask yourself why you want to use the private conversation system in this way.
A private conversation is not moderated unless somebody clicks the report button, but if they do the arbitrators will need an accurate record of events. If the posts were editable after the event, I could send you a PM calling you every name under the sun and questioning the morality of your mother, then remove it again before the mods got to see it. You could say you have an email record of the abuse, but I could then counter that you have falsified your email. Posts in the forums are different - many witnesses.
real; hummy.tv is a pretty civilised place (as far as I know, there's no accounting for what happens in PMs) but many other reaches of the Internet are not.
Email notifications of posts in private conversations contain the text of the post itself, and like MikeSh I (and I suspect many others) may well not go and read the actual post other than briefly to clear the unread message count. Suppose you were able to edit the post after the notification email was sent - you have no guarantee that the recipient has not already been and read the post, so even if we allow that the recipient needs to read the actual post, you still don't know when that happened.
Sorry, I stand by what I said before. Without knowledge of the designers' intentions, I can only draw conclusions from the available evidence and those conclusions are therefore opinions, but they are logical conclusions and therefore opinions that should be reasonably universal. If you are really bothered, there is a Xenforo forum to debate it on.