Just random thoughts..[snipped]
I think if MF had successfully hidden HB's body somewhere, never to be found, I think it's potentially possible she could've gotten away with this. With HB's unlocked car door with her purse inside, who is to say that mother and newborn weren't taken suddenly by a random predator? If/when police ever questioned MF, her story was that she had her own baby, born at relatively the same time. She had already fooled all of her friends, members of her own family apparently, and even the fake baby's own father! They all corroborated that pregnancy/baby-story. MF likely thought LE would never have authority to actually check her cellphone location history, or the baby's DNA, or demand proof that she'd given birth, ie labor & delivery bills, testing of her hormones, statement of live birth or something? Imagine if police had shown up, there was no stench to give a clue, and her car was not hiding in the back yard. No sign of HB. Would police have then suspected that MF had killed HB and stolen her baby?
She prob figured she'd get away with just being concerned and sad that her bestie went missing forever, but meanwhile go on to have a new life with her ready-made new family, or at least to receive enough child support to support herself and the child for the next twenty years.