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.
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.
Have a nice trip!