Complex sentence: He discovers that Pinbacker is still alive and has made his way into the "Icarus II" observation room, having sneaked aboard and decoupled the ship from the "Icarus I".
Split into simpler sentences: He discovers that Pinbacker is still alive and has made his way into the "Icarus II" observation room. He is the one that decoupled the "Icarus II" from the "Icarus I".