I agree with you that people have committed crimes in order to get people's attention but ultimately it was not the illegal actions that cause changes to happen. it is when people go through the proper means that is what causes the changes to happen.
You give the example of those that helped slaves, while that was illegal, those individuals were ready to go to prison for their actions. I think that they didn't try and find ways to justify it to avoid prison, they were willing to suffer the consequences of their illegal actions. And if people today who are doing illegal actions for their activism are ready to go to prison, then that is commendable. I am not willing to do that personally but if other people are that is their choice.
Also, you use whistleblowers as an example, for the most part in the private sector whistleblowers are not committing any crimes, in fact, that they are protected by the law, as long as they do it within the law. Government employees are usually barred from releasing classified or sensitive data, but there are legal means for even these people to report to people that can make needed changes. I just believe you have to do things within the law and how the law prescribes.