I think both ideas are great. The challenge projects are a great way to figure out where we know enough and where we need learn More.
The mentor for group it's a really great idea. Someone to help and direct studies. I don't know any one too recommend though.
My time is Also short. I work full time with regular over time. Most of my time for learning Comes on weekends.
Tell me More about how you see these challenges working. Would it like we decide on a pacman clone and we each work on one and compare results?