1. One key point is that if the true orbit is circular the low mass star revolves around a point in the center of the projected ellipse, but if the true orbit is elliptical, the low mass star executes an orbit around the focus of the ellipse. These two situations can be distinguished wth sufficient observation of the motion of the two stars.
2. Add text and diagrams for this solution.
3. For a visual binary the stars have to be separated enough on their orbits for us to resolve them into two stars at the large distance from which we view them. By Kepler's 3rd law, large orbital separation implies long orbital periods.