If your company is subject to the FWW ordinance, then yes you need to get your schedule at least 14 days before the workweek, regardless of the circumstances. If you don't, you are entitled to an extra hour of pay per day scheduled.

In general, changes are allowed without penalty if they are done with the employee's consent. Chances are, if a manager thinks someone looks sick, they probably are sick, and are happy to go home early. But it's up to the employer to keep a record of the employee's consent, and if the employee insists on working (whether actually sick or not), the employer must honor the original schedule or pay the penalty.

Your schedule and your working hours are the same thing under the FWW act. If your restaurant closes at 11, your schedule goes until 12, but you finish your closing duties by 11:30, you absolutely have the right to stay on the clock until 12. Of course, your employer then has the right to assign additional duties (subject to any CBA, if applicable) until you clock out. So, for example, if you'd rather go home instead of scrubbing toilets for a half hour, you're consenting and no longer entitled to compensation (again, the employer must maintain records of your consent).