Is actively reporting wrongdoing considered divisive speech?
Apparently, according to the commentary on rule number 3 of the Pācittiya section of the Pāṭimokkha, one of the defining characteristics of divisive speech is that one has to have the intention of either gaining favor with another party, or causing the disgrace of another. Whereas if one is acting with the genuine intention of putting an end to harm, or to have someone help the other person see the error of their ways, it is not considered divisive speech.
More discussion can be found here.
So, some possible answers might be:

If one does not develop ill will toward the perpetrator of the act of corruption, and one is genuinely interested in preventing or stopping harm, one can report the act to the police and it would not be considered divisive speech. Although the severity of the criminal consequences here raise more interesting questions in this case.
If, by not reporting the workplace relationship, harm would befall the witness or the people involved in the relationship once the relationship is discovered; and, additionally, one does not have the desire to shame the people involved; then one can report the relationship to the ethics body of the company without being considered divisive speech. Since in this specific case, the code of conduct does not forbid relationships, but only unreported ones, no additional harm would come from the report.