Ok, so about Subaru, how about this? The modification to the ship is actually inspired by a big predator that uses ultrasound to find prey. So Subaru originally uses it to map the area and find a shortcut that they have to navigate without it due to the mentioned energy drain. They soon find signs that the other racers are also taking the shortcut, which Kikki mentions it makes sense since they have the home field advantage. Subaru though notes that some of the ships are broken like something rendered, presumably the before mentioned predator. At first they are surprised that they were attacked, since they presumably would have avoided any nests with their knowledge of the area but Subaru realizes that if the predators can sense their own ultrasound to hunt, then they presumably can sense the pulse Subaru's ship sent and so they think that there are rivals encroaching on their territory. Subaru then proceeds to use the pulses to agitate the monsters into attacking all racers and wreck the underground changing the layout, which nullifies the home field advantage of knowing the paths while updating Subaru's map with every pulse.