Adding recipients to the bottom of the mail, combined with adding group names to group PMs, is no doubt the best solution to the topic at hand; “Email Notification recipients unclear when PM is sent to multiple users”. I think both of those UX elements ought to exist, regardless of whether or not we expand upon our mailing list parity.
Personally I’m not too enthused about the “multiple duplicate reply keys” mailing list feature. I’ve always found it confusing and I’ve seen people mess it up multiple times. And mailing list users are predominantly technical.