Advanced online bus ticket booking

Hello everyone. Our family of 13 people will be travelling to Osaka in June. From Osaka, we wish to take a night bus to Hakone. I found 2 bus but going to Odawara. That will be fine as we can buy our Hakone Free Pass at Odawara Station, then off to Hakone Yumoto from there.

This is where our problem is.
Willer has one departing Namba Nankai at 22:10 and arriving Odawara at 5:45 next morning.
https://willerexpress.com/st/3/en/pc/bus/route/calendar.php?oR=1806003...
http://www.nankaibus.jp/express/10.html
And Kintaro bus has one departing Osaka at 21:35 and arriving Odawara at 8:13 next morning.
https://www.kintetsu-bus.co.jp/en/express/routelist/route_numazu_gotem...

1) Upon clicking further into the reservation, we found out that some requires payment at convenience store. But the biggest problem is they limit the number of tickets per order. One says 4 and the other says maximum of 6 tickets. How can we buy all 13 tickets at one go as we don't want anyone to get left behind?

2) Also, the advance booking can only take in booking of one month prior to departure. As we are coming from out of Japan, one month is very risky and no guarantee that we will be able to get all 13 tickets at one go.

Any solution will be most helpful. Thank you

You can send e-mail.
https://willerexpress.com/st/3/en/pc/contact/

I did found out that Willer sells that route but it was actually Nankai Bus that operates the Osaka-Odawara route. So I tried writing directly to Nankai but the reply came back was to come back one month before our date to do the booking. Very short and simple reply and mentioned nothing about the 4 tickets limit per booking.

Then I also wrote to Japan Bus Online but the reply came back was the same as Willer's.

Have anyone ever made bookings of more than 10 tickets through the online bus ticketing before? Any advice will be greatly appreciated.