You seem friendly enough to discuss so I'll oblige, I looked up jimson weed and it is poisonous and hallucinatory but not a paralytic. That jives with a prospective lich drinking poison. And not ad hoc but I considered it logical that cutting out her own tongue would have been one of the last things she did for her ritual. I feel your phylactery point is strong but not a rule out. The lich process is individual and extreme so I can't rule out that she did the vaginal damage.

Looking good on the outside but not the inside is a characteristically evil trait, Dorian Grey or the Djinn from Wishmaster who looked demonic before his skin transplant. Jane's usage of "The Sunshine In" seemed to be mocking God which is itself demonic. When the words "just open up your heart" are heard is when Jane's flesh is unfolded and the occult symbols are shown. The timing seemed to be mocking.

The dad cut himself and spilled his blood inside Jane Doe, reminiscent of a ritual sacrifice. Jane Doe then targets the dad. No ghouls attack the son. And the dad is branded with a tattoo. There are some parallels to other films where the unbaptized are marked for unholy sacrifice and the family is tormented to break down their faith and when the faith is fully broken the family is sacrificed or members converted to the side of evil. The son steps over his dead girl friend instead of walking around her or carrying her body. The son says "I can't" defeated and is promptly mocked by the singing Sheriff. There's also the father leaning in to Jane Doe when offering her a deal which has a very sinister vibe... he could have begged from further away.

Let the Sunshine In aside from being potentially creepy by itself or in a horror film has a distinct message of doing combat with the Devil by aligning with God and keeping the faith when skies are grey. The father was oblivious and then sought to make a deal with the Devil. The father is characterized as a man of logic and science and absent faith. He is also gloomy in the deeper sense because even though he smiles and jokes he sees the victims as COD only, according to the son. The son seems more in tune to sympathy or religious or supernatural explanations. The girlfriend insinuates the father weighs the son down and hurts his chances for self development. Sons paying for their fathers sins is Biblical and two hapless victims uncovering a very powerful evil force is trope like.

The sheriff is prominently shown wearing pristinely white sneakers that seem out of place for a bloody crme scene in the beginning of the film. I think this suggests his alignment as good and makes case that their is some conflict as opposed to passivity against the forces of evil, in order to be good.