This chapter confirms that 15 points rule is only for Kurono. The other players would have been more desperate and trying to steal kill if they had the same rule. Props to Izumi for leaving the "hidden" door open, accidentally or not. The bike helped a lot in the mission.
I doubt not having the suit gives points multiplier. In the bird aliens arc, Kurono earned 38 points after taking out 6 birds, and 1 boss bird without his suit. Each bird is worth 5 points based on Kato's score. If Kurono had a point multiplier, he would've received far more points.
It also kind of makes sense since Kurono takes out the Bronto boss, and the boss' kid as well as 2 T-rexs and a lot of raptors. The Brontos are probably worth a lot of points.