Which rail passes are best

Hello,

2 of us are traveling to Japan in May:

These are the train trips we need to take:
1. Saturday May 6 from Narita airport to Tokyo.
2. Tuesday May 9th from Tokyo to Onomichi.
3. Saturday May 20th from Kyoto to the Osaka airport.
Note: This is 15 days from start to finish.

Questions:
1. Should we buy a JR train pass in advance for 14 days?
2. If yes, can we buy a one-way pass on day 15 from Kyoto to the Osaka Airport?
3. Is there a more efficient way to take these trips? We are on a tour from May10th-20th

Thank you.