Please see below for the answers to your questions.
1. You can buy the highway bus ticket even on the departure date. However, the bus from Shinjuku to Kawaguchiko gets booked so quickly, so I would suggest that you book your ticket in advance from the following website.
However, the bus leaves almost every 30 minutes, so you should be able to get a seat if you wait a little bit.
2. If you miss the boarding time after the purchase of the ticket, you have to buy a new ticket. You cannot get a refund for the missed bus.
3. No limits. However, it is a first-come, first-served basis. So, you if there are not enough space left in the bus compartment, there might be a limit. If you have lots of luggage, I suggest that you go to the bus terminal as early as you can.
4. Fuji Q Highland drop off and the Fuji Q station are not so far from each other. It is a walking distance.
Hope you have a nice trip in Japan!
Have a nice day.
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