Any question answered...

I'd assume you can only edit it before the recipient had read it for the first time.

A question for @Velocio unless you want to review the source code.

On this forum, if I edit a pm a couple of days after I sent it, does it edit the message at the recipient’s end as well as mine?

Yes.

And you can always edit things.

However, a full and complete history of all edits of all private messages is retained until all parties to the message (sender and all recipients) have deleted it.

So if you think there's been anything illegal or dodgy, then it is technically possible for me to verify the history of messages.

I have never had to do this, not even when I've worked with police to ensure convictions for bike thieves... but it is possible by virtue by my accessing the server and using the applications secrets to read the messages directly from the database.