The grantor of the POA is the only one that may revoke. Given his dementia, he has not got the mental capacity to revoke. And, if you step back and think it over, the fact that he chose his daughter to be POA before his state of mind diminished, his wishes are in place as is.
You have gotten what you are entitled to get via FOIA. The police do not need to release investigative documents further than they have done.
Just because the SA has not prosecuted so far does not mean the SA may not do so pending further investigation. It is precisely because the police do not want the manner or method of their investigation to be public that FOIA is executed with this balance of interests in mind.
Keep looking for a lawyer, remember to not speak to the other insurance company and make sure to fill out the accident report requrired to be filed with the Secretary of State.
Most important, though, is for you to continue regular medical treatments to get yourself well. Do not stop your course of going to your doctor.
Keep looking for a a lawyer.
It is time for a lawyer, not a website, to discuss what claims you may have. AVVO is a free general legal information blog, not a substitute for legal advice. Look for an attorney you would like to have a consultation with. You can use Find a Lawyer tab above, right here in AVVO. Select an attorney, contact them directly and schedule a consultation. You might be glad you did.