OK, so I see you have very strong feelings about this, lol. BTW, I am very glad your little guy survived! What a thing!
My theory--working theory that is, and it can change--is this:
Mom drinks. A lot. JI hates it and tells her she needs to knock it off. She says she will, but keeps on drinking. They quarrel over it sometimes. He is sick and tired of dealing with her drunk a$$ night after night and having to deal with the kids at bedtime and everything else because she is getting tipsy by then. He doesn't want to kick her out because then she will take the baby and he *knows* what a custody struggle will be like, from past experience. And she might get full custody and keep on drinking. Better he keep her there where he can at least keep an eye on the situation.
He is a little worried about working that evening shift because he doesn't know what she will do while he is gone. She promises she won't get drunk, and he trusts her and goes ahead with the night shift because they need the money.
She tells JI she needs to go to the store just to buy some baby food and wipes. She pays with the joint debit card for that, and borrows money from her brother to buy the wine that she isn't supposed to have. That's why he got the change. She'll pay him back later. She hides the wine somewhere and JI takes off before she brings it in. He has no idea she plans to get drunk that night and she knows he would be furious if he saw the wine.
She goes ahead and drinks with her neighbor, figuring she will be fine by morning and he'll never be the wiser. She'll get the neighbor to throw the box away at her house, or just hide it somewhere.
Later, after the neighbor leaves, Lisa wakes up and is soiled. DB decides in her drunken state to go ahead and give her a quick bath then give her a bottle and put her back to bed. She draws the bath and puts the baby in and gets distracted or nods off. She comes to and realizes the baby has drowned. She knows JI will totally blame her. She can't let anyone know what happened because for sure JI will toss her to the curb with her son and she will have no place to live and no way to support herself. She has burned too many bridges and has little in the way of family/friend support. They may not be rich, but it is pretty comfy and she doesn't want to lose it.
She has to come up with a plan to hide this terrible thing she has done. She is not thinking clearly. She wraps the baby up and tapes her well and drives down to the river and throws her in along with the phones. It was an accident, but her husband, and probably the police won't see it that way. If she calls 911, the result will be either jail or homelessness. Either way, the baby is gone, and calling 911 and telling the truth won't bring her back.
I haven't fully figured out the phones yet. Obviously something is on them that she doesn't want anyone to see. That's my theory du jour. Go ahead and pick it apart and let's see what works and what doesn't. You won't offend me. Just be nice.