Salil, the step page class is always an SR (strategy result) class. Price and Points in your example are indeed 2 properties from the SR class.This is exactly what we are trying to explain in this lesson, that the strategy propagates this SR class from one component to another. Check the Auto-run results as well.
The applies to class of the strategy is indeed the Customer class. Whenever we need properties from this class we can access it with Primary.<property>.
Absolutely. When you create a new strategy you either need to specify a business issue and group, in which case the SR class will be the proposition group class. Alternatively you can provide directly an SR class.
Once a strategy is created you can not change the SR anymore, unless you do a Save-As.