You do not pose a legal question for us to answer. Nevertheless, it certainly is possible for a head injury to cause a change in mental status as well as new physical symptoms/problems. Whether a diagnosis is proper is a question for doctors to answer. Hope all of this turns out well for you and that you make a quick, complete recovery!
Please advise what the substance was; how it got there (your best guess if you do not know) and how long it must have been there. Also, please advise whether you have the names and addresses to your fall. With this new information I will be able to give you my best answer in accord with FLORIDA LAW.
There is no Florida law requiring an adult to wear a seat-belt in the back seat of a car.
At trial, the only way that not wearing a seat-belt could impair your claim is if the defense is able to prove that you would not have injured your shoulder had you been wearing that specific lap belt.
For pre-suit settlement with an insurance company, this may present a serious problem. Your credibility regarding the matter will be the real key issue- did you tell all your doctors that you...
Classic "he said/she said"- how does one side prove they are right? Your witness testimony seems useless since you wrote there WAS an impact and the witness didn't see it.
With proper insurance, your auto insurer should handle this entire matter through conclusion and you should have nothing to worry about. If there was fraud involved, your insurer will go out of its way to investigate the claim. Insurance companies love fraud cases.
It isn't your job to interpret the medical records to...