I am already of the opinion that Misty cannot tell time. Or not very well. I think she knows the hours-3 o'clock, 6 o'clock, etc but not the quarter of or half past. 6:30, 3:15 etc. I also think she does not know measurements-feet, inches, yards. I think she can cook, but cannot probably read 'measurements-1/4 cup, 1/2 cup. 1tsp vs 1 tbs. The family she grew up in certainly did not give her cooking lessons, and her inability to do math is probably a learning disability, diagnosed or not. This very well is why she couldn't learn, school was hard and she stopped going.
Therefore. I don't give too much credence to 'she went to bed at 10:00..or 10:30...or 11:00. it is very likely was she went to bed at 8 or 8:30, right after the kids. Either she didn't feel comfortable saying she went to bed that early, or she glances at a clock and reads the big number...I don't think anything based on Misty's time perceptions will be accurate.
I think she has a memory for 'things'...but not details. She may know Haleigh was dressed, but not what she was wearing. She may know it was cold, but not the temperature. I don't think Misty is lying as much as she has limited cognizance and memory. Details have probably been relatively unimportant to her, and she never trained herself to remember them. She reminds me of many special education children I have worked with with learning disabilities. I feel like they paint the world with a large paintbrush, but cannot use the smaller brush for details. She probably was perfectly capable to watch the children, run a washer, cook a simple meal, and put them to bed. But when an emergency came up, she would be unprepared to handle it. She probably slept the sleep of a child herself, not anticipating any danger (doors were locked) and sleeping deeply til Ron got home and the 'world was well". She leaves out important details because details are unimportant to her-they don't barely imprint in her mind. When asked about these details, she is probably trying to remember and fill in these gaps, but doesn't appear accurate as she may be 'creating' them, or again, didn't notice them well in the first place.
I don't see deception, as much as I see confusion. She is overwhelmed, and probably realistically has 'no clue'. Myself? I do feel she was home sleeping-whether drug or alcohol induced, or just the dead sleep of a child. I honestly believe she really has no idea of what happened. Failing the lie detectors could be because she 'thinks' she knows what 'may' have happened. It could be fear, and it could be worry-in my opinion, she has some guesses of her own. It may also be the way the questions are asked-for accurate answers, the questions would have to be asked in a manner and language she understands. Someones mind would have to understand the way her mind works, and ask accordingly. Perhaps someone local in the area who is used to teaching or working with special-needs children and learning-impaired children could question Misty-they may get better results, phrasing questions in dialect and words in a manner that are simple yes or no answers, or questions that address the way Misty's thought processes would work.