1) Would you consider Lunges to be a Compound lift?
2) When I do alternating BB lunges, my lower back feels REALLY worked out too. Do you think it's normal?
My opinion: yes to both questions.
Yes and Yes. I switched to DB lunges, my back doesnt feel it at all anymore.
If you focus on keeping your upper body as upright as possible, it won't hit your lower back as hard.