I don't think you should rush yourself just because it's a collaboration with some other artists. Being honest to your collaboration partner is the key to any successful and long lasting collaboration. It seems to me that you still have no clear direction into what the concept of the artwork is going to be. Because the reason of you having difficulty in shading is not that you can't shade, but rather you still haven't grasped what you're shading. Take the time to clear out the idea of the concept first, because that's the heavy part. After you've done that, you probably will have easier time to finish the artwork.

Hey Jonas, Adding shadows from imagination is pretty hard. The best way to do it is to pick a light direction and intensity. Then just use logic to place your shadows, and how dark they should be. Also, you have some proportional issues that I addressed in a draw over. I hope you find it helpful :)