Should I get the pass?

Hi there,

1.) Yes, a 7-day JR Pass will be good value for travel from Day 4 to Day 10. The cost of a return ticket Tokyo - Osaka/Kyoto is similar to the price of a 7-day pass. In other words, any additional travel you cover with it, is basically free.

2.) Seats can be booked in person, at any JR ticket office, see:
https://www.jrpass.com/blog/reserving-seats-boarding-the-bullet-train

Kind regards,
Daniel