Trains passes&reservations

Hello! I am still a bit confused by all of the trains. I think I already figured out that the JR Pass is not for me, so I need to buy area passes, and I still have a few questions - mainly about reservations.

1. NIKKO PASS
I found out that I can buy them conveniently through Klook. But I do not want to redeem them yet; instead, I would like to reserve a seat in advance. How can I do so? (I'd like to make a reservation sometime around March 23).

2. JR Kansai Hiroshima Area Pass X JR West ALL Area Pass
My itinerary looks like this:
1-3 Kyoto, 4-5 Osaka, 6 day trip to Nara (from Osaka), 7-8 Koyasan, 8 Koyasan-Hiroshima, 9 Hiroshima-Miyajima-Kanazawa, 10 - Kanazawa-Shirakawago-Takayama, 11 Takayama-Kamikochi (optional) and to Tokyo.
I have already decided that the JR Kansai Hiroshima Area Pass will probably be best for me, even though I am traveling after the change that will be made for the JR West All Area Pass (including the JR shinkansen line from Tsurunga to Kanazawa). However, I still have a lot of unanswered questions:

2.1. JR West ALL Area Pass - Shinkansen between Kyoto and Shin-Osaka is not included. Do I need to get off the train? Or can I buy this particular route in advance, and how?
2.2. Google Maps shows me the route from Hiroshima station to Kyoto (Takaido-Sanyo Shinkansen) and from Kyoto to Kanazawa station (Limited Express Thunderbird). For the JR West ALL Area Pass, the only part not included will be Shin-Osaka to Kyoto, right?
2.3. How can I make reservations in advance?
2.4. How can I buy train tickets in advance? If I will take only the JR Kansai Hiroshima Area Pass, I would like to continue from Shin-Osaka further without any stress.
2.5. Where can I make reservations for buses: Kanazawa to Shirakawago; Shirakawago to Takayama; Takayama to Kamikochi; Kamikochi to Takayama?

3. OVERALL questions regarding transportation
3.1. What are the usual rush hours?
3.2. I read that if you do not have a reservation, you can wait for more than 2 hours to get onto the train. As I planned our trip during the highest travel peak, I would like to be prepared.

Thank you very much for your help. I really try to find everything on my own, but I am still a bit confused.

Shinkansen between Kyoto and Shin-Osaka is not included. Do I need to get off the train? Or can I buy this particular route in advance, and how?

If you travel from Miyajima to Kanazawa, you don't need to take Shinkansen from Shin-Osaka to Kyoto and then "Thunderbird" limited express. Instead get off at Shin-Osaka and take "Thunderbird" to Kanazawa (or Tsuruga where you transfer to the newly-opened Hokuriku Shinkansen). Shin-Osaka to Kanazawa is fully covered by the pass unless you take Shinkansen from Shin-Osaka to Kyoto.