That much oil dumped on your MB
There is a good chance it came out of a recycle bin/tote from just about anywhere. I would not be surprised there is more in the dumped waste than motor oil. That could explain why you are having such trouble.

When dealing with the insurance, I would point out you don't know for sure what it is, but it looks like motor oil is part of it, and whatever it is, its ruining your car.

Brother, we feel your pain