What is the ruling on working in a pharmaceutical manufacturing plant, with the possibility – albeit unconfirmed – of harm to the patient resulting from erroneous practices during manufacturing, and what are the financial guarantees that ensue from that?

What you mentioned in your question does not involve cheating, deception, or negligence. Those responsible for extending the validity of certain raw materials are specialists. They act according to scientific standards and the laws regulating these matters. Therefore, your work does not involve any violation of Islamic law or any suspicion of being unlawful. You are experiencing obsessive doubts, so you should completely disregard these whispers and pay no attention to them.