Yes, both editor and engine will be open source. I also do a good job of intellisense commenting, so even without looking at the source, every class, method, property, etc. will have a good description of what it does from intellisense.
Beta won't be out for a long time. Maybe 3 months until the alpha, another 6 months until the beta, and another 6 months until release?
Alpha should have the most basic functionality possible, beta should have a good workflow and all the basic tools, and release will have more advanced tools and better usability.
EDIT: added screens of the multi-tab view. For any tab control (including the ribbon and documents), you can hold Ctrl or Shift while selecting tabs to select multiple, and the corresponding panels will be displayed side by side!