I watched the Netflix documentary last night - first time I heard about this disappearance/murder. I read through the entire discussion as well. It's one of the most sickening murders I've read about - sickening to watch the documentary to the end.
It seems logical that Dennis Bowman's wife knew from the beginning that her husband murdered Alexis. That is, in part, supported by the fact that she was comfortable with his history of sexually assaulting girls and teens, she provided numerous false sighting after the murder, and she easily accepted what he did. My impression is that there is, or was, a mild version (if such a thing is possible) of
folie a deux between husband and wife. That is, at some point Dennis viewed Alexis from a predatory perspective - perhaps when he returned home from prison. He gradually, in small steps (leftover sandwich) convinced his wife (of lower intelligence) that Alexis should be treated poorly, eventually justifying sexual assault. It's even possible that, after his wife became pregnant, she expected Alexis to step up to satisfy Dennis in order to protect the unexpected pregnancy. At each step towards increased abuse, his wife looked the other way; did nothing, seemed to agree with Dennis' persecution of Alexis.
When Alexis reported the sexual abuse to her adoptive mother and to the school, Dennis' wife was not only of the mindset that Alexis was a liar and a rotten child, but that she deserved what she received. I think that Dennis' wife was entirely dependent on Dennis for her identity. That is, if she went against Dennis, she would lose her lifestyle, emotional support, companionship, moral compass, and balance in life. When she finally seems to understand that Dennis sexually abused Alexis, murdered her, chopped her up with a machete and an axe, and buried her with dirty diapers, she sets that aside and acts as though nothing is wrong - presumably because she believes that it is what Alexis deserved.
His wife seems to have two personas - one for investigators, and another one for Dennis. With investigators, we almost believe that she sees her failings. The next minute, with her husband, we see that she continues to support him. Is that because she can feel pious and guilt free with him?
We see towards the end of the documentary that they are desperate to be together, to be able to regularly see each other in person. My impression is that they need validation from each other to believe that what they've done is acceptable. Dennis reveals the location of Alexis' body only because he expects that, in exchange for that information, he will imprisoned near his wife. He obviously didn't get anything in writing, since he did not get what he wanted.
I do wonder whether his wife always knew where Alexis was buried. Thank goodness for mother's intuition about the location of Alexis' body!
... this is an opinion based on the documentary.