Help with my journey

Hello, in a few weeks, I'm flying to Japan, but a few things are bothering me. If someone knows the answers to the following questions, I would be grateful.

1. Which specific train can I take to Nikko with the Nikko World Heritage Area Pass? I know the departures are from Asakusa station, but does this pass fully cover the round trip, or do I need to pay extra?

2. Does the JR Kansai Hiroshima Area Pass fully cover the journey from Shin-Osaka to Hiroshima and back if I want to take the Nozomi?

3. Is it possible to buy an online ticket for the Hida Limited Express train from Shin-Osaka to Takayama? As far as I know, this train runs only once a day, and I'm afraid that if I don't buy it early enough, there won't be any seats left. If it's not possible to buy it online, can I purchase it at the Tokyo station?

4. Where can I buy an online ticket for the bus from Hirayu Bus Terminal to Shinjuku Expressway Bus Terminal? I emphasize that I'm from Central Europe because I noticed that not every part of the world can purchase certain tickets. If it's not possible online, can I buy the ticket at Shinjuku station?

5. Can I buy an online ticket for a direct bus from Tokyo station to Shimoyoshida station? (around Lake Kawaguchi)

[Nikko]

You can take
Tobu Express (without "Limited") trains and
Tobu Local trains
just with your Nikko Pass,
as long as you are in the coverage area.

To take a Tobu Limited Express train,
you pay the Limited Express surcharge
(1650 yen for Asakusa -- Tobu-nikko one way).

- Japan Guide: Nikko Pass
https://www.japan-guide.com/e/e2358_002.html

[Hiroshima]

Does the JR Kansai Hiroshima Area Pass fully cover the journey from Shin-Osaka to Hiroshima and back if I want to take the Nozomi?

Yes.
Unlike Japan Rail Pass,
this JR-West pass lets you take Nozomi and Mizuho,
with no extra payment.

[Takayama]

JR-West Online Train Reservation covers
the route of JR Limited Express Hida 25,
including the JR-Central section beyond Maibara.
: Osaka -->> Shin-Osaka -->> Kyoto -->>
Maibara -->> Gifu -->> Mino-Ota -->> Takayama.

can I purchase it at the Tokyo station?

You can purchase JR regular tickets
at any JR ticket counter.

You buy at least one JR basic fare ticket.

The route which is to be covered
by your basic fare ticket
depends on your itinerary and time allotment,
i.e. where and how long you stay or travel.

(a) You travel in western Japan
mainly with your JR-West pass,
and make a round trip
from Shin-Osaka to Takayama.
--> Purchase a pair of basic fare tickets
separately for that trip.

(b) You travel by JR beyond Takayama.
--> Consult a JR attendant
about your basic fare ticket.

- West Japan Railway Company [JR-West]
https://www.westjr.co.jp/global/en/
| Press "Reserve" button.

- JR-West Online Train Reservation
| Look at the right side menu,
| then "Purchase Tickets for Specific Sections" link.

- Select Date/Time and Departure/Arrival Stations
| Specify the departure and arrival stations,
| with no extra space before or after each name.
| (The system cannot regard "Takayama " (with one space)
as Takayama Station.)
| Select the departure date and time.
| Un-check "Use Shinkansen" parameter.
| Press "Search" button.

- Select Route and Seating
| Hida 25 appears.
| Press the button to specify the seating.
| Press "Select" button.

- Select Ticket
| Press "Select" button.

- Passenger Information Entry.