Itinerary and JRP activation question

Our plans have changed. We are now flying from Osaka to Sapporo (CTS) and renting a car there for the last 5 days of our trip. This means that we will likely only need a 14 day pass as opposed to a 21 day pass, but I want to confirm that this makes sense and ask when the best time to activate our pass is. Here is our exact itinerary.

8/10-8/15 Arrive 11pm in Tokyo, with 5 nights in Tokyo
8/16-8/17 Train to Takayama for 2 nights
8/18-8-21 Train to Kanazawa for 2 nights
8/19-8/22 Train to Kyoto for 4 nights
8/23 Train to Osaka for 2 nights
8/25 Train from Osaka to Kansai airport

Additionally, we plan to take day trips using the train while in Tokyo, probably two, maybe three destinations between Kamakura, Hakone, Nikko, or Yokohama. Additionally we will make a couple day trips from Kyoto; probably to Nara, and then either Hiroshima and Miyajima, or Kobe.

So the question is really regarding the benefits of having the JRP to get around Tokyo and make the side trips. Presuming we need the JRP on the 25th to get to Kansai airport, then working backwards 14 days means we would activate the pass on 8/12, so we would have two days in Tokyo with no pass and perhaps just pay for a Passmo and maybe one day trip separately (like Hakone) Conversely, we could pay separately for the Train from Osaka to Kansai airport which means we could then activate a 14 day pass immediately upon arriving to cover everything in Tokyo?.

What is your advice and btw, where to I activate the pass when we arrive?

thanks