Are there academic institution-specific equivalents to attorneys (a professional who assists in navigating conduct and justice systems)?
I’m not aware of what you are asking about existing as a type of professional that one can retain (as per your question) — I suspect that would be a rather poor way to make a living — but at my university there are people who are available to consult on matters such as Mary’s expulsion case.
Since Mary sounds like a graduate student, the relevant place to go would be the university ombuds office. In some cases the student union or an employee union of some sort may offer some help.
If Mary were a faculty member, she would go to a committee of our academic senate where advisers would be able to advise her if the thing she wants to complain about is a violation of her rights as a faculty member, and about the procedure for challenging the administration’s decisions.
These advisers are themselves regular faculty members like me and only do this as a “side gig” for a few hours a month, but they will have received specialized training that gives them the knowledge to offer this sort of advice (in fact I am currently on a sister subcommittee of this committee so I received a lot of the same training myself), and will also have auxiliary staff they can turn to who specialize in the relevant policy matters and can offer further information if the need arises.
These advisers won’t be available for an unlimited time. Likely they will be able to meet with Mary 2-3 times and discuss her questions, but it’s not quite the same as having your own lawyer or other professional that you retain on an hourly basis.
Since through my involvement in the sister subcommittee I am privy to faculty grievances a bit like Mary’s, I can tell you that faculty members who really need professional help with their grievance process will often hire a lawyer (I have also seen cases of students hiring lawyers for help with a disciplinary matter), and this can make a big difference - it’s far from easy for someone without experience in these administrative processes to navigate the system. A lawyer may not be intimately familiar with the specific institution’s policies, but broadly speaking they have the right training and skill set to help in such challenging situations. They don’t come cheap, however...