Chin Na are specifically designed to rapidly allow one lock to be changed into another. Extremely low postures can be used, but it is preferred to stay off the ground. Many Chin Na will not work on the ground, because of the need for footwork to provide a pressure source and angle. Practice lacks the fatigue and soreness that comes from being repeatedly picked up and smacked against the ground. While there are many techniques that move an opponent to the ground in a submit position, there are also many techniques that can lock the opponent upright or in bizarre positions.
Spontaneous improvisation is an expectation and a necessity for the good grappler.
You're as good as you are good. Bad as you are bad. Na tends to make the student very flexible, and so has ironic health benefits. The pain stops when the class stops, because the practitioner becomes expert in knowing the border between pain and injury. One set is not better than another.