Shibazakura Liner

I would like to take the direct bus from Tokyo to the Shibazakura festival but would then like to take the shuttle to Kawaguchiko and take the bus back to Tokyo from there. I have seen a lot of information about buying round trip tickets for the shuttle at the Kawaguchiko station but since I am not going there first, my question is can I buy a one way ticket from the festival to Kawaguchiko station at the festival.

You can.
But return bus of Shibazakura liner fare is 1230Y/oneway adult.
http://www.shibazakura.jp/liner/

So, you have to book only direct bus fr Shijuku, and return bus
fr Kawaguchiko sta. to Shijuku, Shibuya, or more in Tokyo.
http://www.shibazakura.jp/eng/access/
http://bus-en.fujikyu.co.jp/highway/
Shinjuku line is the most numbers of operating.
The bus bound for Akihabara is not Fujikyu, see the "Keisei" bus web.
https://goo.gl/hFAHVh