I figure that Misty is very simple plain folk (not an insult in any way shape or form) I have met plenty of people like this and even have some in my own family.
My take on Misty? She's young yes. She might be naive she might be emotionally immature but she's not stupid...she's stupid like a fox if she's like the people I have known.
Is she afraid of him? Might be. I don't think that would keep her from falling in love or having a lover in another man. Would she stay at Ron's trailer with this man? I doubt it. She's not that stupid. If she got caught messing around Ron would probably tear her a new one. If she got caught sleeping with another man in his home or bed...he might kill her. I bet she knows his limits what she can do and get away with.
As for trusting that Haleigh wouldn't tell Daddy if she had caught her and the man somehow. Even though she says that the kids thought of her as a Mom, she's not their Mamma. She has only known them for maybe 5 months? Cared for them for 4 months? In my mind, she wouldn't use that unless Haleigh herself was afraid of her Daddy's anger and what it can bring.
So for today, or this hour actually, I'm leaning towards a lover and not getting out of the house to be with friends. Either Haleigh saw them and the man did something to the child and Misty was witness...hence her "I covered her with a little sheet" comment and tears.
Or, she and her lover were out in the area, or in her lovers car/truck and she propped open that back door and when she came home Haleigh was gone.
The reason I'm going with this is because it is the most simple answer to why she might have not been in that house. But it's just my thoughts as of now and I am probably wrong