What’s my option if I missed my bus?

Hello,

Am now in Shin-Osaka area and we're taking the S Hikari JR train tomorrow to go to Lake Kawaguchiko. However, I missed my deadline of reserving seats on the express bus from Mishima to Lake Kawaguchiko. What other options do I have in order to reach Lake Kawaguchiko? There are three of us. I read that if the bus is not full, we can get on it without reservations But I don't want to take the chance.

It's also possible that we can take the same train which goes all the way to Gotemba and from there take the bus to Lake Kawaguchiko.

Another option is to go back to Tokyo, leave our bags with our hotel, and take the bus from Shinjuku afterwards.

Any advise is appreciated.

In case the direct bus to Kawaguchiko is full (I doubt that this is the case on most days), you can take either a local bus or local trains (via Numazu) to Gotemba from where you can take another bus to Kawaguchiko:
https://www.japan-guide.com/bus/gotemba.html