If it was an accident you call 911, murder not so much.
That's not true. By all accounts, the mother has some mental problems (uncontrollable liar, etc) and it was probably clear to all that an accidental drowning explanation would not be believed.
Yeah she has mental problems, she lied and partied when her daughter was missing. The woman is a psychopath, which lends more to murder.
As the prosecution states, no one takes an accident and makes it look like murder.