Dark WAS a plant... (spoilers)
I've watched this movie twice in the last 24 hours (once alone and then once with my boyfriend after telling him about it.) And after two viewings I've concluded that Dark was indeed a plant. Here's why:
In the introduction we see Deaf applying a bandage to a cut on his head. I think this is an injury from a previous "exam" with a different set of candidates that got out of control. His cuts are never addressed even though the director is very precise in using everything which is shown (the bandaid from blonde's heel at the beginning gets given to dark, the drain shown after the sprinklers goes off is where the pill lands, the gun shown when brunette finds the lighter, etc.)
Our next real encounter with Deaf is after the lights have been smashed and he's whimpering. White takes notice and Dark tells everyone "He's French," even though he hasn't spoken yet. When he does speak he says "Vous voyez la...?" which means "Do you see <the...>" and she translates it as "Can you see?" Then he says "Vous voyez? Regardez bien." which means "do you see? look well." But she translates this as "Can you see it in there?" Next he says "vous meme" meaning "yourself" which she actually translates correctly, followed by Deaf saying "vous vous voyez" which means more closely "you yourself see" or in better english "see for yourself" but she translates it as "It shows you yourself." Lastly, he says "toujours y voire clairement est primordial, meaning "to always see there clearly is essential" which she translates as "to see clearly is all." I think the fact that she knows he's french before he speaks french coupled with her translating his pretty straight forward statements into more cryptic expressions proves she's manipulating the exam and applicants. All he basically says is "do you see the/it... do you see? look well, look for yourself, it's essential to clearly look there (at the paper)" but she adds so much poetic license to her translation that the meaning of his words is lost.
Next we find out she works for HR. It makes sense to me that a member of HR would be present during the exam to control the application process and review the applicants.
When Brown begins interrogating her, he raises her skirt to find cuts on her thigh, which he interprets as self-inflicted due to assumed psychological issues. He then paper-cuts her thigh. I think if we follow the idea that Deaf's injuries are from a previous Exam which got out of hand, it's logical to assume her injuries are from being interrogated in a previous Exam after the former applicants found out she worked for HR.
Lastly, she knows all the rules and knows that talking to the camera will get her disqualified. I think she intentionally disqualifies herself to allow the remaining applicants to sort themselves out, after all, if she is a plant, she can't be the last one standing. Also, I think there being a previous exam (or multiple) is why Deaf spends most of his screen time whimpering/crying. He knows these exams escalate to a dark and dangerous place and he's genuinely scared because he knows what kind of crazy, desperate actions are to follow and Dark (as a plant) is there to continue manipulating the group after Deaf bows out and can no longer manage to manipulate the group (adjusting the timer) He's not a type A like her and the rest of the group and can't be relied upon to stick out the long haul.
Maybe I'm wrong. But I think if we "look clearly" we'll see that there was more going on than just Deaf hanging out in the Exam for no reason and Dark trying to get a leg-up in the company.