Question: Would Zep have faced charges for his role in the game? If so, what charges would he be facing exactly?
Answer: It's possible he could get off due to the extraordinary circumstances of what transpired. But given his sadistic pleasure in the game, I would assume he might face some charges. Among possible charges: kidnapping, assault, battery, weapons possession, criminal conspiracy, child endangerment, attempted murder, among many others.
As a lawyer, I believe the law is clear that you will not be charged for crimes you were forced to commit under immediate distress - that your life or a loved one's is in imminent danger.
To be clear the danger must be imminent - if John had told Zap to do it or he'd be poisoned next week wouldn't count, for example. But since the poison was already in his bloodstream his threat of death was indeed imminent, thus preventing any charges should he have lived.
Question: I've watched all seven films, but I've always come back to the same question: How did Jigsaw, Amanda, or Hoffman get some of the keys into their victims' stomachs via swallowing? I mean Nina has a fishhook in her stomach with the key attached! And Donnie Greco had a key stuck in his stomach before Amanda decided to gut him like a fish. Since they were sedated, swallowing keys sounds very unlikely, since it is a conscious effort, right?
Answer: It's shown in Saw 3D (saw 7) that this is all made possible because of Dr. Gordon.
Answer: If you can show that you were forced to commit the crime under duress, then you may be able to form a defensive strategy. Even if this cannot get you acquitted, you may be able to plea down to lesser charges and receive a lesser sentence. When you are arguing for the duress defense, you must show that: you had a reasonable fear of death or serious harm; you did not have any way to escape the situation; you could not have contacted the police about this; you did not force this situation to occur.