If you drag a mail folder to the Finder, it will create an mbox file. You can do that for each [sub]folder and then drag those back into a new profile. Since the db repair util isn't seeing anything wrong, but still seems to be carrying over something corrupt to the freshly-rebuilt db, dragging out to mbox files would certainly strip any Entourage overhead, so it's worth trying.
Also, what happens if you copy that same profile to another Mac? I wonder if there's a font corruption or something else specific to that machine outside of Entourage, that isn't showing up in your checks (maybe very obscure). I assume you've also tried killing all Entourage/Office prefs, etc, and recreating the account (POP3/SMTP) settings.
