Is it ethical for a graduate student to provide juniors with paid graduate school application guidance?
I think you have largely hit the nail on the head in your own post. There is nothing intrinsically wrong with charging for valuable experiential information. In fact, this really could be an extension to charging for 'edits' or tutoring. Really if they are providing non-sensitive information regarding the procedures and what the admissions panel look for - then I don't see any major ethical quandaries.

However, you highlight a key point. They could be leaking information explicitly obtained from the panel (that I assume they would have delivered in confidence). If this is the case, then I imagine it should be intuitive what the schools reaction would be. The reason this is unethical (from my POV) is that they are no longer sharing experiential information, but providing them with explicitly 'leaked' outcomes that would give a disproportionate advantage.

By analogy with the edit example. It would be fine for a tutor to edit and provide feedback on a students work. To the majority of people, I think they are aware this occurs and there are no ethical problems. In fact, a tutor might go so far as to show them example assignments. In fact, my department actively encourages students and postgrads to seek and help out students, respectively. However, if the tutor provided the marking key (obtained from another postgrad running the unit) and gave this to the student - that is blatantly unethical.