First of all, sorry to hear you are in this situation - sounds quite unfair. Also, sounds like you may a cause of action against them, but I think you need an attorney to do some more research on the organization that administers the exams and what their policies. A lawyer should review the letter you received along with the paperwork you filled out when registering for the exam.
I am no expert in this area, but know that tests have been invalidated before for irregularities not caused by student error. The MCAT is a private business and they employ the proctor, unless you had reason to know you were being given extra time and did not say anything to the proctor, and/or it was a grossly excessive amount of time, I would challenge it. I know similar situations have happened before and these testing companies insist they have this right, I would hire a lawyer and sue them - assuming the contract you sign for the test does not agree they have the right to void tests even if not your fault. Someone will need to read the contract and regulations carefully,
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