Questions about JR Planning

Hello, I will be in Japan from the 6th to the 21st of December. And I have the following Questions:

1) When should I order the JR Pass?

2) I'm going to be staying in this order, at the following cities: Tokyo, Kyoto, Osaka, Hiroshima, Yokohama and from there for to the Narita airport and back to the U.S., what are good cities to visit outside of the cities where I will be staying, so that I can get the most out of my JR Pass and start planning my itinerary. For example, which cities are near Tokyo that are worth seeing and are accessible by JR Pass and can be seen in one day? Which cities are close to Kyoto, and so on and so forth.

3) The JR Pass is good for 14 days but I'll be there for 15, how much am I gonna spend to get from Yokohama to Narita Airport?

Thank you so much!!