Amanda and Daniel are the only ones in the house who aren't coughing or showing other signs of dying. This hints that they have already been given the antidote.
Amanda yes, Daniel no. Daniel is seen sporting the rash on his neck when Amanda tells him about her incarceration and he's coughing and wiping blood from his mouth after he and Amanda find the bathroom from the first film. When Eric finds Jonas' corpse, the safe is open and there's an empty needle on the floor. That was from Amanda giving Daniel the antidote after Daniel killed Xavier. But it's definitely fridge brilliance on the Amanda front.
When I saw Saw II, I was annoyed at how the ending cut off so suddenly. Sure, Amanda's Face–Heel Turn/backstory reveal was cool even though I already knew who she'd be (although I thought she turned between the second and third films and was wondering "If he's there [in the safe], where's Amanda!?" so it wasn't a complete waste of a good twist ending), but it seemed a little hollow. A while later I realized: The house wasn't Jigsaw's game, and Amanda's game for Matthews wasn't for him to escape before the tape ran out, the whole film was the game she had set up for him! The "game over" was much more satisfying once I realized that Jigsaw's game hadn't led him to Amanda's trap, but it was entirely Amanda's trap and he'd already failed!
To add to that, what made the twist of Amanda being Jigsaw's accomplice really work was the realization afterward that you should not have been remotely surprised if you actually thought about it for a moment. She claims that she's being punished for being "naughty", with a flashback scene of her cutting herself. But as is revealed at the end of the film, with the same scene shown but with a wider shot showing Jigsaw rooting her on, a man whose entire philosophy is based on proving through bloodshed that you value life would hardly have condemned her for such an act.
I saw that as her giving herself the scars to make her story convincing. After all, Paul was placed in the Razor Wire Maze because he cut himself (although he only did it for attention).
Except for the fact that as she's cutting her wrists while John watches, there's a voice over of him saying, "In order to beat death, you must be reborn." And since John's initial test for Amanda was to give her shock therapy into quitting being a junkie...It does make sense (until the reveal) that "Jigsaw" would be pissed that after giving her a drug-free second chance, that she "squandered" it by attempting suicide. Her main attire throughout the film also seems to be setting up that she's been confined to a mental hospital, since she has no belt or shoelaces to finish the job, making it seem like "Jigsaw" angrily plucked her from whatever institution she was recovering at to test her again. It's a tough call. Both theories apply.
Also, as noted in the DVD commentary (and above), not once during the entirety of the Nerve Gas House trial did Amanda show symptoms of the gas such as the coughing everyone else did.
Initially, the Razor Box seems to be an inescapable trap. However, if one looks at the top of the shot as it pans higher, you can see the lock holding the top of the box closed actually has the key already in the hole. If only Addison had taken a closer look around, she could have avoided a painful and gruesome death.
The Needle Pit's room evokes a Nightmarish Nursery aesthetic with the childish wall sketches and scattered dolls. This might make a parallel to the trap's drug theme; Xavier, a drug dealer, is meant to dive into the needles (presumably used for heroin consumption) down the hole, and he forces Amanda, a former heroin addict, to do the work for him instead. Drug addicts' need to consume drugs frequently can be compared to that of a baby or young child requiring attention and care.
