Hi,
first of all congratulations for your project, it's something really useful. My CMS currently uses TinyMCE and has tight integration with it, so I would not be happy to replace it. After all, it offers lots of features, and can be configured to be quite XHTML+CSS compatible.
I found interesting applying your CSS to TinyMCE: could it be enough to achieve the intended result? See a screenshot at http://www.cromoteca.com/my/wym_mce.png