Does cannabis not only increase likelihood of spirit possession, but cause alteration of brain function chemically that worsens the spirit degradation of mental health, and leading to serious mental illness?

You are close to the truth here, but it is still the case that the spirits are the causal element and have the upper hand here, so cannabis use weakens the aura and greatly increases the risk of spirit possession by these dark beings. Once onboard, the dark spirits begin to undermine their host, and in those individuals who are susceptible to such manipulation, they can wreak havoc in causing distortions of thinking and, over time, create a permanent alteration, meaning that it takes on a life of its own. Even if the spirits are removed, the residual alterations will be self-sustaining and permanent without a significant healing effort mounted to undo them. The secondary effects of the active ingredients in cannabis, namely, the psychoactive components, in the way they perturb consciousness and other workings of the mind and brain, will exacerbate the alteration of function induced by spirit meddler attachments. So rather than alter the brain itself directly, in a way that becomes irreversible, they can worsen the influence of the dark spirit alterations to brain and mind function and hasten production of serious mental illness symptoms and a chronic state of mental imbalance.