which JR Pass to use (14 or 21)?

Hi there,

We are planning a visit to Japan and I have some questions regarding the JR Pass coverage and profitability.

We arrive at Kansai int'l Airport and, after 18 days, depart from Narita Int'l Airport

Here's some rough itinerary:
- arrive at Kansai airport and travel to Kyoto (stay there for couple of days) - travel from Kyoto to Nara and back (one-day roundtrip) - travel from Kyoto to Osaka (and stay there for 2-3 days) - travel from Osaka to Hiroshima (and stay for 1 night) - travel from Hiroshima to Miyajima (and stay there for 1 night at ryokan) - travel from Miyajima to Hiroshima and from Hiroshima maybe to see Kyushu (still have no idea what exactly :-)) - travel to Tokyo (most likely by Shinkansen from Nagasaki/Hiroshima) - to spend some time in Tokyo for several days (3-4 at a minimum) - travel from Tokyo to Narita Int'l Airport

To sum up: this is very rough itinerary, and my question is not IF but which one JR Pass to choose 14 or 21 days?
I do know that there's other Pass for use in Kansai area (including local buses/metro in Osaka & Kyoto) so the first few days we might use that and start the JR Pass on our travel from Osaka to Hiroshima

Last question: Would the JR Pass be valid for travel from Tokyo to Narita by Narita Express? I've tried to search the Internet but on most website there's only information about travel from Narita to Tokyo

many thanks in advance and please do let me know if there are any clarifications needed