Buses from Hiroshima to Okayama

I need advice of a resident of Hiroshima or Okayama .In Saturday morning November 14th, I go to Hiroshima from Osaka night bus and immediately going to Miyajima.At midday from there I go back to Hiroshima and see sightseeings (half day). The same evening, I want to go to Okayama by bus.
Questions:
1.Do I have to pre-book a bus ticket from Hiroshima to Okayama or I can buy it before boarding the bus?
2..Where can I buy a bus ticket?
3. What is the schedule for the evening buses from sites (in Japanese)
1).Ryobi bus 両備バス http://www.ryobi-holdings.jp/
2).Chugoku JR bus 中国JRバス http://www.chugoku-jrbus.co.jp /
3).Hiroshima Kotsu http://www.hiroko-group.co.jp/kotsu/

4.Where are boarding places?
5. There is the same shedule Ryobi bus, Chugoku JR bus and Hiroshima Kotsu (old shedule,English)
What does it mean?

P.S.
I can spend only 1 day to see Miyajima and Hiroshima and I want to reach Okayama that day.
Unfortunately it is very difficult to understand on the websites of the bus companies, since the google translator does not translate text from pictures (Project Naptha is on)
Help,please.
Best Regards,Ivan

1. and 2. You can reserve online or by phone, or pay aboard the bus if you have no reservation and there are free seats.

3. I don't understand this question, apparently you have found the schedules since you have noticed that all three companies have the same...

http://www.ryobi-holdings.jp/bus/kousoku/r09hiroshima.html

4. In Hiroshima, you board at the Hiroshima Bus Center, which is near the Peace Park. https://goo.gl/maps/bmpCFh4idGy

5. This is because several companies operate this route jointly, sort of like codeshares with airlines (this is very common for highway buses in Japan). So the actual bus you will take may belong to any of those companies, but other than that it makes no difference, the price and everything else are the same.