Yakushima Hibiscus ferry + Shiratani Unsuikyo

Hi, I'm planning a trip to Japan this November, which includes a day trip to Yakushima (all I had time for!). Initially I had planned to take the Jetfoils either way from Kagoshima, but doing more research into the Shiratani Unsuikyo hike has led me to believe I may not have enough time if I catch the first Jetfoil in to arrive at 9.45am, and the last Jetfoil back at 4pm. I will rent a car from Miyanoura Port, but still think I'll be pressed for time as most sources say the hike is a five-hour round trip. With this in mind, I am considering taking the Hibiscus ferry to Yakushima and returning on the Jetfoil. My questions are:

1. Has anyone had experience with the Hibiscus? Is it hard to get to the port from Kagoshima (I know it's further away than the Jetfoil one)? How comfortable is the overnight stay on the ferry?

2. Has anyone done the Shiratani Unsuikyo hike? How long did it take? I'm fairly fit and can walk far (up slopes too) at a constant pace without tiring too quickly, but I would like to experience the forest properly. Are there any points at which I could stop and turn around if I run out of time, while still being able to experience the part of the forest that inspired Princess Mononoke (I know that's further into the hike)? I know Japanese people are very structured, and like to do walks/runs/hikes in an anticlockwise fashion, so I don't want to walk the opposite way on the track and annoy people!

No one has any information on either question?

Has anyone done the Shiratani Unsuikyo hike? How long did it take?

We visited Yakushima last Oct.
We are prone to seasickness , so decided to fly instead of taking the JETFOIL. Also airfare was only about 1500 yens dearer, as we booked months in advance.
Upon arrival, about 09:15, We picked up the rental car (just opposite the airport) and headed for the Shiratani Unsuikyo immediately .
It was drizzling at the time we arrived at the park entrance ( 10:45 ); in fact the rain got worse for the next 2 hours during our hike. Section of the park was closed due to heavy rain, so we were not able to enter that area. We took our time, and exited the park around 16:00.
It would be nice if you are able to, at least, stay one night in the island.

Thanks for the reply! Yes, I would love to stay a night on the island, but I definitely do not have time. I might consider flying there, but the airport is a further distance from Shiratani Unsuikyo than Miyanoura Port so I'm not sure I'd really save any time, which is my objective.
Shame about the rain! I hope I get lucky with the weather.

For past Jetfoil passengers: How long before the Jetfoil leaves do you have to be onboard/waiting at the port?