Is an expiatory oath (kaffarat yamin) obligatory upon me after swearing to inform the senior official and then informing the junior official, and was the informing necessary, and is it permissible to inform after the time of the problem has passed?

The validity of an oath depends on the al-Ḥālif's (oath-taker's) intention. If, by the phrase "al-Masʾūl al-Kabīr" (the senior official), you intended to report the complaint to officials generally, then your oath is fulfilled by informing the junior official. However, if you intended to inform the senior official specifically, then your oath is not fulfilled unless you personally inform him. Thus, the oath refers back to the intention of the oath-taker, whether it aligns with the apparent meaning of the لفظ (wording) or contradicts it.