How to reserve bus seats for Fuji-Hakone Pass

Hi guys!
I am going with two more people next April 2019 (14 to 16th) from Shinjuku to Kawaguchiko first (9 to 5).
We want to buy the Fuji-Hakone pass, cause it mostly fits our requirements. However, as it will be during Hanami time, we are afraid the seats may be full, and don't want to ruin our plan because of this.
Do you know if we can reserve the bus seats so, when we go to Shinjuku to buy the pass, we can ensure that we don't run out?
Then, was is the easiest way to go from Kawaguchiko station to Hakone Sengokuhara Prince Hotel in Hakone by bus using that Fuji-Hakone pass? Do we need to book the seats in advance too?
We really appreciate your help.
Thanks!

I don't think there are any online reservation tools for pass holders. You can make reservations at the bus terminal in Shinjuku. I do not think that the buses will be excessively busy on regular weekdays. It may be better to avoid weekends.

As for buses within Hakone, they don't allow for seat reservations. Between Fujigoko and Gotemba, you can take either the regular, non-reserved bus or the faster limited express bus that requires reservations, Again, you should be able to make reservations at the bus terminal (Fujisan Station in this case, I guess). Also, I am not sure whether the pass covers the supplement for the limited express bus between Fujisan Station and Gotemba.