it's cool to read you did so much already I'm first interested since I'm also dealing with a community mainly based on a Yahoo! Group I'd like to switch to Tiki, and the direct link with Tiki is not working so well yet.
As you may have noticed on FuturePartners, phpList was listed as the future app to interact with Tiki. However I don't think it's definitive and it just needs some discussion. How does Sympa stand in front of other mailing list apps? Tell us a bit.
You'll want to at least come get a quick first discussion on IRC and then send a mail to tikiwiki-devel.