1) why do we add salt to the ice?
Salt decreases the temperature which helps it cools easier.
2) why do we add the cookies and other things, only after the milk has frozen slightly?
It will make the frozen milk tastier.
3) what are two factors that affect the freezing you think?
Ice and salt
4) why do you need a bit of air in the bag?
In my opinion, the air inside will be the space when milk vibrates and air can change the temperature inside easier.