Hi there,
You can take a Fujikyu bus from Kawaguchiko to Mishima Station (1.5 hours, 2260 yen one way), then take a direct Hikari train on the JR Tokaido Shinkansen from Mishima Station to Shin-Osaka Station (two hours, 12,000 yen, one direct train every two hours).
Just for your information, the link below explains very well about the transportaion between Kawaguchiko and Osaka.
http://www.japan-guide.com/e/e6905.html?aFROM=2157_6900
As for a night bus, all the transportation is quite safe in Japan.
So, taking a night bus from Kawaguchiko to Osaka is not a bad choice.
The one way trip takes about 10.5 hours and costs 8,700 yen. A round trip ticket is available for 15,700 yen.
http://bus-en.fujikyu.co.jp/highway/detail/id/9/
Have a nice trip!