IMO
Theory that Misty was pregnant - just as a few posters thought - and her 3 day binge was not a binge but possibly under medical supervision. Ron has a way of making young girls pregnant and history always repeats itself. If she was pregnant and went away for a few days, the family would know this and be very protective. If anyone caught wind of it, Ronald could be charged with sex with a minor. He married her just in case somebody figured it out. Hence, why Ron was not angry with the "missing for 3 days" and his family is so loving to her (no more grandkids out of wedlock). She comes home after Ron picks her up since she is not supposed to drive. He picks Haleigh up at the bus Monday - since she is not supposed to drive and probably medicated. He leaves the kids and Misty alone that night and Jr watches tv in the livingroom, Haleigh is put in the tub or possibly given medication to make her fall asleep because Misty is exhausted. Misty falls asleep. Wakes up and Jr is watching tv and Haleigh is dead - drown or from the medication. Misty wraps her in a blanket, wet if from the tub, or gets vomit on it if she was sick. Misty transfers her to a sheet and a plastic bag. Props the backdoor open like she does when she takes the garbage out. She leans the bag against the dumpster. She runs to get the van and uses the van to dispose of the bag. Leaves the back door open to look like an abduction.
Or she calls someone to bring the van to get the garbage bag for her.
Check phone records.
Why would she be under medical supervision for 3 days?