The rental agency is related to Frank
It is more than a coincidence that Tess and Keith so happen to book the same home at the same time. They both receive little assistance from the rental agency furthering their suspicion. It could be that the rental agency was very well-aware of what Frank was doing either because he had relatives working in the firm or that he probably worked for them at some point. Since the Mother is trapped underneath the house and is feral, the one way to keep her from escaping would be to provide her consistently with "children." The mother is infertile.
Frank hoped to keep his twisted dynasty going by impregnating her, and lashed out when he found they couldn’t conceive. In her mind, finding and caring for new “children” will keep her safe from Frank’s wrath.- The fact that she’s producing breast milk indicates that she’s been pregnant in the past. Her child/children were most likely stillborn or died shortly after birth due to deformity, possibly starting when she was very young. It’s an especially tragic thought that she became fixated on mothering because her babies kept dying, for reasons she probably couldn’t understand.
The Mother is a metaphor for America’s treatment of women, historically and in present day.
The Mother, like America, is the product of a long legacy of atrocities, particularly toward black and brown people (making Tess the ideal protagonist).Despite the fact that The Mother feels like she is caring for Tess as a mother should, she is actually putting her in great danger and could potentially kill her at any moment (though she seems oblivious to this reality). Similarly, just as the US government feels, sincerely, like they are doing what is in the best interest of women across the nation, they are oblivious to the fact that they have become the boogeyman by enacting or rolling back policies that may well result in great danger and, ultimately, death for many, many women in this country.
Frank’s middle or last mame is Carlos
It would explain why his neighbors don’t bat an eye when he wears a maintenance uniform with the name tag of Carlos.Keith would have pressured Tess to have sex the second night
If Keith had not been murdered by The Mother, he would have been more forward with Tess that night, expecting payback for being such a "Nice Guy" to her. It would have played out similarly to AJ's story of AJ and his co-star.