If the pain is intolerable, you are well within your rights to seek care at the ER. No one should have to tolerate that level of pain untreated. (By the way, my daughter was just diagnosed with Chiari 1, she gets headaches, but I believe they are unrelated in her case.)
What you are suffering from sounds very similar to my situation, but I have no possible diagnosis for you. i just wanted to post to reassure you that you were heard. I'm sorry you are suffering and I hope you find some relief. I would definitely advise you to see a neurologist who specializes in headaches. Your pcp can make a referral. If possible make your appointment for after your MRI so the neuro can get the results.