If you can stand it, I would bite the bullet and return to the original sketch.
The answer to your purple shadow question is almost certainly yes. To avoid flattening the form, you should remember to always highlight within your previous highlights. Do not expand the next level of highlighting beyond the previous (see belly in attached paintover). Also avoid highlighting near the edge if it's the only light in the scene. Lastly, make sure that when you're refining you don't change too much, like making the belt wider at the top or one of the pecks much larger than the other. Set boundaries, and try your best not to cross them. It's definitely a very will-testing process the first few times, but is necessary.
Oh and, from a composition perspective, maybe give the creature some breathing room at page margins (especially the top and right sides). It's not as pleasing to have him cramped in there so tight. Dangerously close to creating a tangent on the top, and already creating one on the right side with the toe.
