If sub-tabs don't work, then I would suggest you look into developing JS that will provide your desired functionality. If you find something that will work while maintaining flexibility, I encourage you to extend the WDN scripts and commit.
Regarding the rows of tabs - technically you can have as many as you want/need. However, the tabs do not visually wrap well. After a certain threshold the simplicity and value of tabs disappears (not to mention the load required by the massive amounts of content).
User Experience Architect