A) it is not a workaround, that the way that dreamhost feature was designed to work.
B) in your original post you didn't mention mobile devices using pop3 (apop is password security protocol for pop3 that was originally introduced by RFC 1460 ). I wonder why you use pop3 on mobile devices, I use IMAP on mine.
C) if I had to set up email system for users where all email had to be archived for 12 years (legally as you say) I would most certainly not leave it to the integrity of the user not to delete an email (new or old). Instead I would move to a journaling compliant email server such as microsoft exchange. If I had to come up with an email journaling workaround that worked with dreamhost I would create a second account for each user that the user has no access to and use panel enabled filters to forward a second copy of the email to the second account. That would be a workaround for journaling
D) I'm not sure why you are getting to the point in the month in the first place where all of the sudden the inbox archiver wants to expire so many messages. On the only account that I use that feature it seems to expire about 8-12 messages per day very consistently.
Good luck to you, I won't offer any more suggestions.