I think you say your keys flew? So perhaps they are saying you lost them deliberately. I would keep expressing the danger you were in. The fact that you are vulnerable (We all our if we qualify for enhanced mobility) the involvement of the police. And that this is then lost then stolen not deliberately left in a car. They are almost certainly looking at the reasonable precautions clause in your policy. The involvement of the police is definitely on your side. You took what precautions you possibly could in the circumstances from what you’ve said.
This was my career once upon a time on a larger scale but the principles are the same. Good luck.