Answer to Question 1
Agitation promotes the formation of numerous small nuclei that are necessary for a smooth, velvety texture. In addition, the agitation encourages the fat globules to partially disrupt their membranes and rejoin with similar disrupted fat globules to form a larger mass, which promotes a creamier sensation. Agitation also aids in incorporation of air, which increases volume.
Answer to Question 2
Crushed ice is better than cubed ice due to the increased surface area, but cannot freeze ice cream alone. Rock salt is used for homemade ice cream preparation because it helps keep the canister and its contents cooler. An ice and salt mixture is used to remove the heat from the ingredients in the canister so the ingredients start to freeze.