You're not at the end of the line yet. Getting the date changed may be not be an option, but there are still steps you can take before going full Edmond Dantes. First, I'll try to answer the more general questions.
How can students know when they are being treated unfairly?
You usually know at a base level when you're being wronged. You clearly were on multiple occasions.
How can they get their complaints taken seriously and appropriately escalate issues through their department/college without being ignored?
Being respectful and professional is key. The second you "rage," you will no longer be taken seriously. Avoid personal attacks at all costs and always check yourself before you go forward. Assume that people are reasonable and willing to help, even when they seem otherwise. Communication is about the message that's received, not the one that's sent.
Carefully measured accusations of impropriety can sometimes help, but be cautious, they will also burn bridges. I had a strong disagreement with a professor where at one point he refused to discuss the topic further. In turn, I summarized our interaction and accused him of being unprofessional. When he defended himself, he included a restating of his understanding of my goal. At that point, it became clear he mistook my request for my objective. What seemed like arrogance and dismissiveness were really misunderstanding. We were able to resolve the matter and are now on good terms.
Judicious use of carbon-copying. CCing the wrong person at the wrong point will actively work against you on the points of both respect and professionalism. That said, there is a role for it when transitioning between administrative levels. When you CC someone in on an ongoing conversation, they can see the back and forth history.
If you are:
- Clearly conducting yourself properly and respectfully
- Being treated unfairly
- Have a concrete goal they can do something about (simply complaining won't cut it)
then you can attempt to escalate the matter to the person's direct superior by CCing them instead of generating a new e-mail chain. You should be immediately switching the direction of your conversation to the new person at the same time, referencing the fact that your are doing this because you are out of options, and be in the right. You will be very, very heavily implying misconduct on behalf of the previous person in this case, but will want them to come to that conclusion on their own. One could make an exception on this in extreme cases regarding ethics, safety, etc.
I think you are past the point where you can begin to CC someone's superior before taking it to them directly, as that requires some degree of personal familiarity. That does remain a potential, if very risky, option in other situations.
Is there any recourse for them - a way to get the school to correct the issue?
There are three further levels you can take this.
The President of the College. The individual College Deans still have to report to someone. You do not want to telegraph your moves anymore at this stage. I don't have a good suggestion for the best way to frame your request for an appointment, but you can't afford them asking the Graduate College Dean for details prior to the meeting if they've already been compromised. Here are some more important tips for this:
- You will also have some serious convincing to do that you're an adult. Be wearing a suit/dress and be immaculately groomed.
- Have your greeting and synopsis rehearsed.
- Lead with the impact this has had on you, not the history; your career and licensing is also their primary method of exposure to the working world.
- Have every single piece of documentation you can scan or print, and have it ordered chronologically and separable by party at fault, if possible. Chronology is more important, as it shows your body of support and could reveal any collusion against you.
- The copy of documentation you bring with you needs to be something they can keep then and there.
The Accrediting Agency. This is no longer about degrees at this point, and it will do nothing for you directly. I'm not even sure the issues at hand are relevant to them. But they do vouch for the integrity of the school. You will really just be filing an official report against the college at best, but if your degree dates remain changeable by College policy and they refuse to do so, it could be considered a form of academic misconduct.
Legal Recourse. (Disclaimer: I Am Not A Lawyer) You could go the legal route, as suggested. They are damaging your ability to earn income in an easily quantifiable way. Legal fees, lost wages, and maybe some psychiatric care may be able to be recouped, but you will certainly be paying up front for this. This will probably cost you more money overall, not to mention the time and stress dealing with the court proceedings.
As you take this matter to increasing levels of aggressiveness, you will find your support waning. People don't like to get involved with other people's problems, especially when it ceases to benefit them. I personally recommend stopping after the President regardless of the outcome. Your health and happiness are the most important things you have, and this is already taking a sizeable toll. You really have to ask yourself, in twenty years, will you be happier that justice was exacted or that the whole thing is a distant memory?
Finally, find a good therapist. If nothing else, it's a place you can vent where the negativity won't linger. Your home should be where your soul can heal and rest, not where you dump the wastewater from washing the wound. I hope something in here is helpful, and wish you the best of luck.