The SureFire XVL2-IRC Pistol & Carbine LED Light and Laser Module System will mount to picatinny rail. It can be used on an AR15 or rifle with picatinny rail space.
Yes. The SureFire XVL2-IRC Pistol & Carbine LED Light and Laser Module System is designed to fit to picatinny and universal accessory rails. So it will fit your HK45CT and function as intended.
You will need an adapter.
Yes, but I think you’d be infinitely better served by going to route of an IWC mount + Surefire M600DF (or Surefire Vampire if you need IR as well) + laser module like Steiner’s TOR Mini. It’d be dramatically more powerful alongside being much cheaper. This is meant for a pistol or other very small firearm where size and weight are a big concern. There are better options for a long gun.