GMail seems to be re-timestamping email messages copied via IMAP, so IMAP is not an ideal way for me to import old Outlook messages, as I had thought. All the old messages appear new in list views and sort new. D'oh.
I added an update to the original item.
Update: Based on a comment from Stephen Foskett and some workarounds/fixes I read about in the GMail Google Group, I am hopeful I can get this working in the next few days. Jamming at work right now, though.