Hmm, so the original issue was that Thundebird didn't send the message when you thought it did?
AquaMail asking for which attachment to keep if editing a draft pulled from the server is normal.
So is the presentation of new / old content, split into an editable area with the existing draft content below. The existing content is not indented and no quoting header is added (e.g. "WHO at WHEN wrote:").
The original problem seemed to be that Thunderbird did not send the message, but it had been saved in draft. That of course is NOT an issue for aquamail. It IS probably an issue with the automation of the hightail add-on.
Not sure if you are familiar with hightail (originally called yousendit ). If you try to send a huge attachment, 10 Meg, for example, Thunderbird will offer you hightail. The first time, it installs an add on, I believe, then afterwards just offers to use it when you use a big attachment. In the attachment panel, it turns the hightail "attachment" into a placeholder, with a hightail logo, instead of a file icon. Doubtless this placeholder generated the "unknown" attachment AM found. A link is also placed at the end of the email so that any client or webmail can find it. Within Thunderbird I think you can just click the placeholder as if it was a normal attachment.
Back to your question:
You are right in saying there is no indent, no WHO at WHEN.
It also looked as you described - during the edit, there are distinct new/old areas.
But as I said, the to: and cc: as well as the subject were already filled in the the top area.
With a forward, I would expect to/cc to be empty, and the subject prefixed with FWD, or similar, but that did not happen.
While I was editing the new line, it otherwise behaved very like editing a message for a forward.
I had possibly 50 to 60% of the screen height to type in, and I fully expected the result to look like a forward, or maybe a lot of white space. In the finished email, there is a single empty line below the new text, then all of the old text.
I had ended the added line with a single newline, before hitting send.
I can pm or email you the screen grab of the sent email as it now appears in AM, but it is about the same in Thunderbird, too. I won't post here as it has personal data (mine, not secret, but not to broadcast round the world!)