Rebound Effect or Mid-Day Crash due to Ritalin Wear-Off: This is the most likely diagnosis because Arian is taking a short-acting stimulant (Ritalin) in the morning, which can lead to a crash in the mid-day as the medication wears off. The symptoms of headache, screaming, and planning to escape the house are consistent with a rebound effect or crash.

Inadequate Dosing of Ritalin: The half-pill dosage of Ritalin might be insufficient for Arian, leading to inadequate control of symptoms and a subsequent crash.
Intuniv and Ritalin Interaction: Although Intuniv (guanfacine) is typically used to mitigate the side effects of stimulants like Ritalin, the combination could potentially contribute to the mid-day crash, especially if the dosages are not optimized.
Underlying Anxiety or Mood Disorder: The symptoms of screaming and planning to escape could indicate an underlying anxiety or mood disorder that is exacerbated by the medication regimen or the crash itself.

Serotonin Syndrome: Although less likely, serotonin syndrome is a potentially life-threatening condition that can occur with the combination of certain medications, including some used to treat ADHD. The symptoms can include headache, agitation, and changes in mental status.
Hypertensive Crisis: Guanfacine can cause hypotension, but it's essential to monitor blood pressure, especially when combined with other medications. A hypertensive crisis, though rare, would be a critical condition not to miss.

Medication-Induced Psychotic Episode: In rare cases, stimulants like Ritalin can induce psychotic episodes, especially in individuals with a predisposition to psychiatric disorders.
Pheochromocytoma: This rare tumor can cause episodic hypertension, headaches, and other symptoms that might be exacerbated by certain medications, including those used to treat ADHD.