BW> If there were an MN option to exclude messages that are *not* to me,
 BW> i.e. to narrow /P to messages that *are* to me, the global personal scan
 BW> would function just as I would expect it to. So it's very close. =)

It sounds to me like you may want:

Command: MN (Message New Scan)
   Data: /P /G /NOFROM /NOLR /NOREAD

This will do a newscan of personal messages (/P) in all groups (/G) so only messages from or to YOU.  But then it will exclude messages FROM you (/NOFROM), exclude messages you've already read once before (/NOREAD), and then it will not update your last read pointers afterwards (/NOLR).

 BW> What would the recommended ACS fields be for a local private area (local
 BW> "PMs" between users), and a local non-private area (local "conference"),
 BW> respectively?

 BW> Name        ³ Private Messages (PMs) 
 BW> List ACS    ³ g1
 BW> Read ACS    ³
 BW> Post ACS    ³ !fa
 BW> Sysop ACS   ³ s255g1

I think it looks pretty good now but you'd probably want to remove the "g1" from SysOp ACS because it means you'll only be able to edit, delete (etc) e-mails if you are in group 1 (which I am guessing you don't want considering you can read from any group).

You're on the right track now though.  Private Messages will show up as a message base when users are in group 1 and do an area list.  Users who do not have flag A cannot send e-mail at all.  Anyone can read at all times which means MC menu command will work in any situation.

... Just another prisoner of gravity!

--- Mystic BBS v1.12 A47 2021/07/27 (Windows/64)