JR pass questions

Hi there,

My partner and I are traveling to Japan for the for the first time this year, arriving on October 29th and leaving November 15th. Our trip is as follows: Tokyo-Osaka-Hiroshima-Kyoto-Hakone-Tokyo.

We plan to get the 14 day JR pass but not activate it until we leave Tokyo to go to Osaka.

1) Can I purchase the JR rail pass this far in advance?

2) How do I activate the JR pass when I arrive? And If I am not using it until 4 days into my trip can I still activate it when I land or do I activate it the day I intend to travel?

3) Can I reserve all seats this far in advance?

4) Our luggage is 158cm (H+L+D), do we need to reserve special seating for this? How do we prove the dimensions if asked?

5) I also saw people ask about the Hello Kitty Shinkansen, what route is this and how do you book this in advance?

Thanks so much in advance 😊
Emma