Thanks for the response Peter re: the original post.
We are very convinced at this point the issue isn't a technical one. We're not sure however if there's an issue with that duplicate content site we found stealing some of our articles, or as you mentioned a quality score issue. We're approaching it as a need to re-group around the quality issue for now, and monitor results over time. We've identified several areas for improvement in that regard.
This stuff is so frustrating to be honest. I get why Google can't show their cards, but the complete lack of transparency or ability to get some feedback from them makes this a difficult game.
Thanks again for the response, much appreciated.