What were the rights were violated and their explanation during the apartheid?
He who goes to bed with itchy butt, wakes up with sticky finger.
Who would advocate to seek justice in a criminal matter where the defendant's rights were violated during the trial?
This question could have many different answers, but "Civil Rights Attorney" would probably be the most specific. In reality, it's the responsibility of all court officers to protect the rights of all individuals involved in a legal matter. If an individual's rights are violated during trial, the appellate courts are delegated the task of correcting the error.
Apartheid was a law that applied to both black and white people, it was designed to separate the races and both white and black people could be prosecuted for breaking it. During white minority rule, under apartheid, the thing that was denied the black majority was one man/woman vote democracy (the ability to make the laws that govern you).
The Supreme Court does not hear cases. They can only make judgements if the persons rights or laws have been violated within the lower courts. They resolve issues based on constitution and due process. If a person's rights have been violated during trail then the the parties have the right to appeal their case. Many cases however never reach the Supreme Court.