I don't care about the headers getting corrupted or keeping anything intact for forensic purposes. I need to edit a bunch of messages for brevity so that they are more convenient to work with later, while keeping them in the Inbox.
I can think of two alternatives.
You can reply/forward the messages to yourself (after editing the reply). If you change the subject you can "break" the conversation (or simply delete the original message after your reply/forward).
The other is to set up an IMAP client to your Gmail mailbox. Some of those will allow for editing messages already received.