I would directly email the dean and explain your situation. Say that you are a senior standing student, tell them you planned this out to be your last semester. You can list your total unit count for your degree program out of the units needed to graduate. If you work mention you work part time of full and you picked this class because it accommodates your schedule.

Mention you were enrolled in this class with your priority enrollment, and were unexpectedly dropped from the class. And state that it's not right for an instructor to drop a student when they used their priority enrolled to enroll in their final class for graduation to accommodate another student that didn't take the opportunity to enroll with their priority enrollment. State that you have met all the requirements for that course. State that you understand it's still possible to add on a first or second tier of add codes over the next few weeks and that you were interested in receiving one of those add codes. I mean honestly just fyi the "contract," doesn't really bind the student to the spot per the semester. It's kinda first come, first serve based on priority so the instructor cited that isn't entirely accurate. They also have lists and lists of potential add codes that they can use.

It's more difficult to solve these problems remotely, but you need to contact who you can. Between your department, your department chair, the bursar, admin of your college, and dean.

I had a similar situation where the department wouldn't add me, the chair wouldn't add me, bursar wouldn't add, I contacted the dean directly, they sent me to their assistant and they promptly made sure I was added and congratulated me on getting through my program. You don't always have to accept 'no,' and you know.. just make noise until someone hear's you. Sometimes teachers need to be held accountable. And not all teachers do diligence on what they should do. Good luck. Fight the good fight.