Polysytrene factories and hypothetical health concerns
It is good to see the some authors want to build realism into their fiction: this clearly isn't common. And you have done extensive research already.

The problem is that industrial chemistry is very heavily regulated. So, while it is common for dangerous substances to be used, it is rare to find places where they can plausibly be misused. And the standards for harmfulness are very low. Even some things at the harmful end of the official scale are not that harmful.

The biggest problem you will have when it comes to realism isn't really to do with polystyrene manufacture at all. A factory that makes products with polystyrene won't usually make its own polystyrene but will buy in the plastic raw material as polystyrene beads which can be mixed with other ingredients (like colours) and melted in moulding machines which produce the product. So none of the chemicals used to manufacture the plastic will be used anywhere while making products from the plastic.

If you were to assume that the raw polystyrene was made locally (which isn't that realistic) then you might find plenty of chemicals that could be misused as you have already realised. CCl4 is one of the best candidates but i rarely used any more. Benzene is used in manufacture so is also plausible; styrene (the monomer that is used to make the plastic) is also nasty and possibly the best candidate you haven't considered.

However, if you want to stick with broad realism and a story that uses plausible things associated with a jewel case factory, then you need to consider a different approach. But it might be easier because you can make some things up that even experienced experts can't question. You could, for example, imagine that your factory adds some special ingredient to its products to create unusual colours or effects (maybe the cases glow in the dark, are unusually strong or contain inks that allow patterns to be printed deep into the plastic by lasers). The ingredients to create this effect could be anything you want (and you probably don't have to be too explicit about exactly what it is: give it an obscure acronym). This ingredient could plausibly have any bad properties you want without being unrealistic. Possibly the product will have been invented recently to give the products a special edge and nobody else has yet observed the side effects.