How do you lead with the elbow when throwing forehand?
Starting with your arm reached back, just keep the elbow tight with your hips without trying to bring your hand forward. Your hand will follow your elbow, thus you will lead with your elbow. This adds an arc when your hand forces itself around your elbow, adding power+spin just like in a backhand. Backhand gets an extra arc because the wrist can curl and whip, but that is minimized in forehand.
Yup. Keeping the elbow tight to your side while rotating your body is "leading with the elbow." It's not quite as noticeable as with backhand, but I think it's more natural to people who have played throwing sports.