By "Reference" Do Hiring Companies Mean Also Credit/Criminal Checks

Simple answer - you don't. You trust that the law is being followed and that IF the job was rescinded due to something on your credit or criminal records, the college would follow the law and say so. You don't jump to the conclusion that the college is acting illegally because they won't give you what you want - i.e., an answer that in the very large majority of cases, they are not required to give you.

You are mis-using or mis-interpreting what is meant by retaliation. If the temp agency gave you a negative reference because you reneged on a job (for whatever reason) that is NOT illegal retaliation under anyone's definition.

I think you need to get your head around the fact that this job is gone, that you are not going to be given a reason why, that the law does not require that you be given a reason, and move on. I know it's disappointing; I know it's frustrating. But continuing to beat your head against the wall is not going to do anything but give you a headache.