You make a very good point.  I just cant think of any other reason the window would be open in winter.  Or that they would have the scene with the backpack.  The show is very deliberately made.

I think the kid grabbed the backpack and went out the window. 

Legally she is not guilty of anything.  She didn't act to save someone.  Not a crime in this context.  Involuntary manslaughter requires a duty to act. She didn't have one. 
But for Alison the humiliation of people knowing what she did (didn't do) would be worse than prison. 

I don't think your right but I hope your right.   I really love Helena.

That argument is idiotic and wouldn't last have a second in court.  Also, their bigger concern would be not having every asset of their origination seized as evidence for the criminal trials.
But this isn't real life. Its a show about clones. If you can accept the existence of human clones created more than a decade…