Corrected entry: In the final scene when Nick hides from Billy under the bridge, Frank catches up to Billy in the bus and brings him aboard. Frank doesn't show any surprise or concern that Nick is missing, despite the fact that they were supposed to leave on the same bus trip.
Corrected entry: In the final fight scene after Rutger Hauer kills the main henchman by knocking him out the window and slicing him in two, the scene ends without anything happening to the henchman's boss. The movie ends without any discosure of what happen to him. He didn't die. He didn't go to jail. He just disappeared.
Correction: He was overpowered and held at gunpoint by Frank. We can assume he was either properly disposed of or left for the police. We don't have to explicitly see it to accept that it most likely happened.
Correction: It stands to reason Nick had already told Frank he wasn't coming with them, and just hadn't figured out how to tell the boy.