The thing is in most states, absent a law that says you can’t do something you can do it.
For instance in Illinois, there is no law that says an employer has to pay an employee for being “on call.” Therefore an employer can hook you up to a beeper 24/7/365
Private work agreements are not usually enforceable as most work is “at will” and can be changed. It may give you some negotiation room if you quit and seek unemployment. You can collect unemployment if you quit for good cause.
For instance, in one company we moved our bookkeeping from a 9-5 shift to the overnight shift. We figured instead of hiring an overnight person to sit around, we’d have the overnight guy do bookkeeping as well. Basically combine the two jobs.
We offered the bookkeeper who was working 9a-5p the overnight job. She refused. She quit and filed for unemployment. Our company fought it claiming she was given another job at a higher shift wage differential. We lost. The unemployment arbitrator said, the working conditions of overnight are substantial enough to be “just cause” for quitting.
In every place I’ve worked where we did payroll what people failed to realize is salary people never worked 9-5 Monday through Friday, even if they actually did.
We put them in the payroll system NOT as working 9am - 5pm / Monday through Friday, but as 5.471 hours per day EVERY day of the week, Sunday through Saturday. So technically salary people worked every day, even though the reality was different.
The problem the OP faces is massive unemployment. You quit there are only far too many people, probably more qualified ready to take your position. You have little room for negotiation. If you try to get “pushy” and make them stick to the letter of your first employment agreement be prepared for backlash. They may well give you those 8 hours but then they will make darn well sure YOU live up to the agreement in every other way.
Just wait till the first day you’re in at 9:01am instead of 9am. As Greg Brady found exact words are hard to live up to. 
As long as employers act in a way that is not legally discriminatory, they are pretty much in the drivers seat. They can change anything to YOU provided they don’t do it for a discrimintory reason. They can change your hours worked if they want to bring you into line with other workers but not 'cause you’re Chinese or Black.
If you’re unhappy I suggest you update your resume and start looking for work. Three things will happen. You’ll get a new job. Or you’ll find out there isn’t much out there and learn to accept your new lot or you’ll quit and learn to take less but be happy that you proved your point to the company and quit.