The kore system sounds like a number of aftermarket 6 bolt to centrelock adapters.... which due to the holes in the rotor being slightly larger than the studs result in a fair bit of play.
Re. Tool, cassette one is about £8 vs £1 for torx bit but both assume that one possesses a socket set to use each one. With practise one develops a feel for how much different sized bolts can ne totqued before they snap.
tis true but the rotors remain the same, just different lockrings required for 15 or 20mm axles
me I'm a fan of centrelock, just simple & it works, as for any play in the system - hopefully you'll be mostly braking in the forward direction so shouldn't really be an issue