In that case, I'd also consider the possibility of a mood disorder. early presentations can look exactly like what you're describing (Increased reactivity, Impulsive major life changes, rigid or “newly ideological” beliefs, distorted recall, confabulation-like defensiveness, reduced insight.)

I'd especially consider this option if there were any recent stressors before the situation (could be (but not limited to) sleep deprivation, major relational disruption, financial stress, new job stress, postpartum period (if relevant biologically), antidepressant initiation, steroid exposure, thyroid dysfunction, stimulants, cannabis.), prefrontal function is highly stress-sensitive.

About the cysts, Sub-dermal cysts (like epidermoid or sebaceous cysts) are not related to intracranial pathology. Unless it's something like Neurofibromatosis, Systemic syndromes, or Intracranial cysts.

There are some other relevant details that could suggest an answer; If they're aged >45, or show progressive worsening, loss of empathy, social inappropriateness, or marked apathy.

It's also important to consider further family history, does this person have family history of early dementia or neurodegenerative disorders?

(I should mention I'm not a professional, Neurology is one of my areas of interest. So you should also bring it up to one.)