Department not allowing me to take courses from another department even though everyone else is allowed
You seem to be good at looking up university policy and rules, and reading them carefully.  So I suggest your next step would be to look up procedures for appealing academic decisions.  Also, get to know your university hierarchy.

Universities vary in how decision making is done, so it’s impossible for me to be more specific about your university.

But I will give you a trick that I’ve had good success with in a variety of educational institutions: Phone or visit the secretary of the highest-level dean you can find that sits above your department, and ask his or her advice for where in the hierarchy to bring your problem.  This secretary is often the most competent, knowledgeable person about university hierarchy in the whole institution.

And a general tip: Tell your story concisely and with a neutral tone of voice.  Begin by stating the problem.  Initially, leave out the numbered list of crazy reasons you were given (but be prepared to provide them if they ask you why the department rejected your proposal).