Numbness/light pain in my left pinky and ring fingers, and side of hand. Maybe spreading elsewhere in hand. Fear of motor neuron disease.

Don't think so. Hi. What you're describing would be a sensory neuropathy, and given the distribution (ulnar nerve), I suspect it's a physical problem ,not metabolic. Motor neuron disease (aka ALS) only affects nerves to muscles, and leaves sensory nerves alone. I'm not a neurologist, but I feel safe in assuring you this is not motor neuron disease.
Suggests following. Anatomy is consistent with a left ulnar neuropathy, but on left arm in your age group, there may be concern of cardiac referred symptoms, and best to have your doctor check for both. Unclear to me why you would focus on "motor neuron disease", which is completely devoid of sensory issues. Would some counseling be of value.
Ulnar nerve. You are describing a sensory nerve issue, not motor. It's possible to have compression or entrapment of the ulnar nerve at the wrist or elbow. Sometimes the issue is up at the neck. See your doctor to figure it out for you!