Sorry if this is a dumb question, but doesn't this open up the ability alter e-mail history at will? And so alter e-mails you'd received and/or sent?
Is there a record of this somewhere? What effect does this have on using e-mails as evidence in court cases, for example? Or is there a technical detail that necessarily gives this away?
Yes. It should invalidate the DKIM signature though, which mainstream servers add to each email now. So it's possible to prove the contents of that email haven't been tampered with... assuming you have a record of the sending domain's public DKIM key. I imagine your email provider would have logs indicating you modified an email too, but I have no evidence of this, nor how eagerly they would dig up such logs for a court if they have any.
I have no idea how people deal with this in practice in court. (What if the domain's DKIM key has changed? Is there an authoritative source of old DKIM keys for most domains?)
And, in the minority of cases where it really matters, and where they really suspect foul play, them I'd assume that they rely on numerous bits of technical evidence (proxy copies, CC'ed copies, file system forensics), plus on one person's testimony vs another's, because as you say, DKIM's usefulness is limited.
There isn't. As per the article, the application creates a copy of the original email, sans the attachment, then deletes the original.