@DeanMarkTaylor since the reply would only go to one user, not sure we need to sort out the multiple users in email response issue. It would be the same approach though–recipient would be identified with a reply key for the PM.
I do think showing the parties to the message in the email is important (critical for multiple party PMs) and trying to match the online experience for PMs would be good at least for consistency purposes:
Not required for this feature since there would only be two users, but would love to fix the “show multiple parties to PM” issues.