Yes there is Mini Shuttle bus, but you can not book it in advance. Its mostly for the visitors who stay here and book it in the ryokan for the next day. There is no other way to prebook the seats. Or you just come and hope there ll be seat for you available.
I can help you organise everything if you stay here, so please contact me on my email.
 

Hello Mickey,
from middle of April till  middle of November you can take direct bus from Yudanaka-Shibu Onsen to Kusatsu (it takes 2hours, costs you 2500yen)
Kusatsu is across the Shiga Kogen mountains.
During witer this mountain roud is closed and you have to travel around the mountains - Train to Nagano city -> shinkansen to Karuizawa -> bus to Kusatsu (it takes 3hours, costs you aprox. 5000 yen)

Yes, there is online Train table on main page of Zeno`s Guide -www.yudanaka-shibuonsen.com/yudanaka_time_table

The roud will be closed for public from 1st. of December until end of April. You can not drive car there. You can take Snow Monkey shuttle bus, which runs from Shibu Onsen parking place to the Jigokudani Parking place during this time. Only this shuttle bus has got special permition to drive this roud. The time table for this bus is not ready jet.As soon as local Tourism assosiation get ready all these information for this Winter, I will uploud on my Guide.

Good decision, I would do the same. Yudanaka - Shibu - Shiga Kogen isconnected by buses and there is no problem to travel up and down.  Evening and nights atmosphere in Yudanaka-Shibu Onsen is very japanese and there are public hot baths, restaurants, bars, shops, games.

Thank you Chun. Yes, there is bus operating between all of this areas I am describing. You can take bus from Shiga Kogen to Kambayashi (30-40min). From Kambayashi you will follow the signs to Monkey park - 30min walk. There are taxi drivers too, but I think its much cheaper by bus.

There is local bus from Yamanouch to Kusatsu. BUT in the Winter time (Dec.-March)one roud in the mountains is closed. During these months you have to go there around the mountains - trough Karuizawa and it takes few hours than. Yudanaka-Nagano city (train)-Karuizawa(shinkansen)-Kusatsu(bus). If you don't want to travel too much and don't have enough time you should give up one place. If you are after Monkeys, relaxing japanese atmosphere, hot spring with not very strong smell, stay in Yamanouchi. Kusatsu is much more turisty, it means lots of people in the streets. I do not like sales persons from souvenier shops are too sticky and want to sell you someting too much many times. Hot springs in Kusatsu have very strong smell and town has also nice old architecture. 

Taxi from Yudanaka to Kambayashi is around 1700 yen. The buses are runing every 20minutes.