Thanks for even more good advice!
Have a Past shooting glove on order, am going to make my trial shoot with slowest, lightest round I could find: B.H. 210 grain, 700 fps "cowboy" load.
Doc who quickly and confidently made diagnosis is my family doctor, not an orthopedic type. That, plus flunking the DeQuarvains test administered by the therapist, leaves me still a bit skeptical.
While discomfort ranges from upper part of thumb to a bit below elbow, it's stiffness in thumb that sticks out (no pun intended) in my mind as being most significant symptom.