Glad to hear the book is helping.
Looking like a good start! Really like the concept drawing, too.
Whether or not hats or shoes or whathaveyou is attached to the model is really dependant on the game engine used. Lighting can break apart where geometry intersects, so in such cases, the mesh is made as singular as possible. In others, it's not a big deal.
For a demo, I wouldnt worry too much about it and do whatever's comfortable. Once they hire you, they can tell you about their specific requirements regarding that kind of thing.