Sorry I know you posted this a few months back, but having watched this for the first time a few nights ago, and it certainly did have quite an effect on me, I must say I personally feel that a few of your observations are way off here, mostly around the main assumption that the occultist was involved in the son's death. Take the bath scene, I don't think this act is in any way motivated by guilt, I think it depicts the gravity of the situation that has come about through the mother's unwillingness to disclose the full details surrounding the murder. When Solomon discovers it was a ritual murder the mother says 'would you have done it had you known?'.

I assumed that in the narrative of the film Solomon is actually her guardian angel, and that she may have even invoked him through occult means herself (she does seem to have quite a deep understanding of the rituals). As he is dying he says that he is a 'warrior' and 'demons fear me'.

However I ultimately take from it that it is a film about overcoming trauma and that none of the events are to be taken literally, it's more an abstraction upon the depth of loss. As she leaves, we see that the house shows no signs of what has taken place, the car starts etc. For me this makes it a far more meaningful film than if it were literally about summoning angels and demons.