Hiking in Yakushima

So basically. 2 Nights 3 days in Yakushima.
Was wondering about Hiking routes.
Basically we will be renting a car (Assuming there is one available)
But I noticed there isn't any round routes.
I assume the best thing to do is double back to where the car is park.

But is there an alternaltive? Where we can do 1-3 routes and get back to the car, instead of taking one and going back the way we came?

I assume the best thing to do is double back to where the car is park.

Yes, that is the only way.
http://www.yakumonkey.com/p/the-hiking-trails-on-yakushima.html

Is there a way to leave the island early?

From what i experienced, only Shiratani Usuikyo got round trips.

https://files.tofugu.com/articles/travel/2014-08-19-yakushima-shiratan...

For the more challenging trails, however, it's not possible. If you are going to rent a car, have in mind some roads may be closed for private cars on specific times at a year.

As for leaving the island check
http://www.yakumonkey.com/p/transport-to-and-from-yakushima.html

I'd suggest having a plan B if you go by ferry/jetfoil as they may be cancelled due to the weather. The jetfoil gets canceled more often than the ferry. Ask the hotel/hostel about the situation.

We got lucky last year because the hut owner knew all but the morning jetfoil would be cancelled. He drove us down to the harbor and we barely made it. Great service :-)