I think there have been some response - but the only real answer is that she states her plans and doesn't back down.
"I have an doctor appointment at 3 today" "I'm sorry, I need you to work at 3 today" "I'm afraid that won't be possible"
"I have a headache and I need to leave" "I can't let you leave, take an aspirin" "I'm afraid that won't be possible"
The boss and crony may never believe the seriousness of the OPs condition. They may always believe she is exaggerating or faking. There is only so much the OP can do about that - her first responsibility is to herself and her own health.
I agree with the above. OP, use that phrase as much as you need to. That's going to be your answer. Your health is vital. And you have to fight to protect it.
The problem is, in some workplaces, this response would get you fired. In many places, absences have to be excused. If a manager said "no, you can't leave," and the employee replied, "Sorry, I have to go," and left, they'd get points or written up. Enough points, enough write ups, and you are fired.
FMLA is a possibility if the OP is working full time. Employers are only required to give FMLA to employees who have worked 1,250 hours in the previous year. They can offer FMLA to part-time employees, but they don't have to.
I'm on the fence about the Dad visit. While I wouldn't want my father visiting my workplace, it does seem as if this visit did some good in opening the top manager's eyes about some of the goings-on in the workplace.
OP, I think you would be best served at this point by scheduling an appointment with the top manager. Don't just go and ask to see him; stop by the office and get his secretary to schedule an appointment with at a future date. During your meeting, tell him that you need this time off, you are willing to give as much notice as possible, and that you are tired of dealing with the attitude from your supervisor. Hint delicately that your next meeting won't be with him, but with HR (I'm assuming this is a large chain store and there's an HR department for the corporation).
Also, contact HR and ask about FMLA. See if you are eligible and if you are, start the paperwork to get it. FMLA can be used for doctor's appointments, or to leave early if you get a migraine. It can be taken hours at a time, or days, or weeks, whatever best serves your medical needs.