Thanks for checking and commenting!
Well, only bindings for rewinding are needed (the unwinding occurs automatically) so only a maximum of 4 keys would be needed.
The ball collects gems because erm... it needs to buy its ticket out of hell? (I definitely totally didn't come up with that just now :P). Lava is avoided because, well, I see no reason why anyone wouldn't want to avoid lava. Especially if you're a melt-prone plastic beach ball (it being a beach ball definitely totally didn't have anything to do with it being one of the only two round sprites in the free Unity asset pack I used).