fresh_eyes, earlier I resisted giving you advice, because you only asked about that UMN lesion. But I honestly can't help myself and came back to throw my 2cents in.
First, those UMN lesions are rather rare. Of course, being the brilliant and intuitive one that you are, you may have figured this arm/hand thing out. But...the pain distribution you described is a very familiar symptom that is reported to me in my medical massage practice.
The most benign (though not necessarily the least painful) of the differential diagnoses are trigger points that originate in muscles in the back of the shoulder (rotator cuff area). They refer pain into the anterior deltoid, and down the arm. Usually, this pain is not accompanied by numbness or tingling.
The second most common reason for the pain distribution you describe is from a compressed or entrapped nerve somewhere along the brachial plexus - sometimes diagnosed as Thoracic Outlet Syndrome. These cases do include nerve-type pain (zinging, numbness, tingling, and severe unremitting pain).
Other causes include shoulder joint pathology (tears, bone spurs, etc)
FWIW, I've yet to have a client with your type of pain that was caused by a UMN lesion.
Omega3-s can help with systemic inflammation. They also clear up the little skin bumps on my upper/outer arm. The only weird side effect I get is that they make my ears very greasy. Beyond waxy, I'm talking olive oil ears
Nancy K. says take 4 grams a day.
The B12 methylation protocol has helped some with neuropathic pain. Have you given that a try yet?
I so wish you were here (or me there) for me to assess you. Dr. Kim always knows when it's a serious condition that requires further evaluation by specialists.