
No, cardio is only one aspect of a fitness program and it doesn't promote lean muscle. Strength training helps with bone density to help prevent osteoporosis and injury.
No, strength training will not make you bulky it will make you leaner and more toned. Muscle is more compact and denser than fat and takes up less space. By the way, the more muscle you have, the more calories you burn.
Lifting light weights is a waste of your time and energy, unless you are injured or recovering. Each set should be challenging. Each set shouldn't be to failure. If you had to do one more repetition you shouldn't be able to complete it.
Weight training will make your muscles, tendons and connective tissues stronger. The stronger you are, the more support you have making it much less likely that will get injured. If you are competitive you will last longer hopefully winning a sprint in the last few seconds of a race or game!