For the archive handling stuff, we've got a couple of issues that I think need to be addressed, certainly for 2.2 if not earlier.
First off, I think there's a problem where the date information is corrected, if you use ARCHIVER_CLOBBER_DATE_POLICY=1. In particular, I believe that this is being corrected as part of the archive process, and if you were to manually re-generate the archives any previous messages that came in with bogus dates would now get "re-corrected" to be the local date/time of the manual archive generation, and not the date/time of message reception. We need to move this code up the chain, so that it will correct messages as they come in, before they get written to the archive, and before they get sent out to the recipients.
Secondly, I think that the code should be modified to look for the most recent "Received:" header, and to use that as a first fallback point to correcting an incorrect "Date:" header on the message. This header should have been put on the message by the local MTA on the Mailman server, so it should be mostly the same as using a localtime system call, except that you avoid any additional delays that may have been caused by the message sitting in a queue, etc.... This also helps resolve the above issue where the message needs to be corrected on input, as opposed to being corrected on insertion into the archive.
See FAQ 3.63, if you're interested.