Both of these refer to particular types of case and agreement. If you use the plural that could infer more than one type.
For example, we could mention all divorce cases where one adopted child is involved - one particular type, whereas there are many differences in divorce cases. In the other example it is only those agreements which threaten market integration and they all fall into one category. There may be hundreds of cases/agreements but they all meet the same condition and become one class.
Student or Learner