Steve says Hopper had a twin brother who was never caught, but at the end of Eyewitness to Murder (1974) (which is shown in flashback), Steve catches the twin Hopper brother while he is checking out of the hotel.
Hopper uses as suppressed revolver to kill Taylor. Revolvers generally can't be suppressed because the noise of the exploding gas escapes from the cylinder, but the Nagant M1895, Russian made revolver was a gas seal design where the cylinder moved forward to seal the gap between the cylinder and the barrel.
When Hopper puts on the glove containing Steve's fingerprints, the camera then cuts to him calling the police, and he clearly isn't wearing the glove.