Why was the boy in the first episode let go from the police station and no one took custody of him since he was a minor. They show him walking out alone. NO social services?

thats generally a problem with lots of dramas. illogical situations.

you will find that in all dramas, like why did a police officer not call for back up, why did they let the family touch a body when it was just discovered, why do detectives get to officially investigate cases where they are themselves a suspect/related to the victims ..... why why why
With some things you just have to find your own peace.

Mouse is a prime A case of drama with lots of illogical storylines and situations. Thus it is either loved or disliked :)