The rules for Dancing Darkness state:
or one dimly lit, vaguely humanoid shape
This does not specifically state that this form has the light reducing radius. I am assuming it does not. However, can the form be passed through? If so and a character were to stand within it would that character be in an area of dim lighting?
No, because the figure is a form of light, nothing physical, it would not provide cover. I can't imagine a vaguely humanoid shape would make it more difficult to hit something in it. Also note that it is 'dimly lit', not 'slightly darker than the surrounding' so it would be more akin to having Faerie Fire on you. If Paizo wanted the humanoid version to provide some specific bonus, they would have expounded on it, as they did the spheres.
What exactly this functions as is up to the GM, there aren't rules about a 'human shaped light' because everything is applied to the baseline, 'ambient' light. A humanoid shape wouldn't fill a square, and therefore wouldn't generate any RAW coverage; however, you could always pitch it to a GM for some fun flavor events, though.
Also, as you noted, it would not decrease light around it because it is clearly separated from the spheres' effect:
each reduce the illumination level by one step within a 20-foot-radius, or one dimly lit, vaguely humanoid shape.
