Aside from the holes in that sprite sheet, most of the errors you point out, Serio, are due to representation of foreshortening. It's not inaccurate when you think of it in that sense. The arm looks skinny to give it depth from the body, and make it look like it's extended away properly. Also maybe the sprite size limit is why they couldn't shade that arm.
In frame 8, the really skinny wrist is due to not wanting to crowd the shape to the body and confuse things.
Just saying.
Anyway, I'm excited to see what this turns into.