Implementing fair work and pay practices and a policy to treat employees equitably and consistently will most likely reduce the pressure or incentive as a common characteristic of fraud. These measures can help diminish feelings of unfair treatment or dissatisfaction among employees, which are often drivers behind the rationalization for committing fraud.
Fraud Triangle and organizational behavior literature.

If an internal auditor believes that the internal audit activity's independence is impaired, the first action to take should be to discuss the impairment with the audit manager. This step is crucial as the audit manager can provide guidance, support, and potentially escalate the issue appropriately within the governance framework.
It ensures that the concerns are addressed promptly and effectively within the internal audit function before reaching out to higher levels of management or the audit committee, maintaining a proper chain of communication and resolution.
The Institute of Internal Auditors (IIA) - International Standards for the Professional Practice of Internal Auditing

When an internal auditor suspects fraudulent activity, such as erroneous payments to a fraudulent bank account, the appropriate course of action is to escalate the concern to the engagement supervisor (Option D).
This step ensures that the issue is handled with the necessary urgency and oversight. According to the IIA Standards, particularly Standard 2060: Reporting to Senior Management and the Board, the CAE must communicate significant risk exposures and control issues, including fraud risks, to senior management and the board. Escalating the concern ensures the appropriate levels of the organization are aware and can take timely action.
IIA Standards, Standard 2060: Reporting to Senior Management and the Board IIA Practice Guide: Internal Auditing and Fraud

The principle most likely violated in this scenario is objectivity. According to the IIA Code of Ethics, internal auditors must maintain an unbiased and impartial mindset in all aspects of their engagements. By considering a job offer from a business unit manager involved in an audit, the auditor risks compromising her ability to remain objective both during and after the audit process. This situation creates a conflict of interest that can influence the auditor's judgments, potentially leading to bias in audit findings or decisions.
The Institute of Internal Auditors (IIA) - Code of Ethics

The control that best addresses the risk that supervisors might conceal accidents to receive bonuses is the investigation of all reported violations. This control ensures that incidents are independently verified and assessed, reducing the possibility for supervisors to manipulate or hide safety data to qualify for bonuses.
References: Best practices in fraud and misconduct control, which often emphasize the importance of independent investigations to verify compliance and enforce accountability.