Did you pay with a credit card? Do you know if this credit card offers rental car insurance? Are they going to just charge you for the fix to your said credit card? You could always dispute the charges with the credit card company.
When ever I rent I pay for their insurance. Many times they decline to do the walk around as one guy said you got the insurance, you can bring the car back in pieces, we dont care.
I never tried it, but I believe the nozzles are made differently so you cant do this by accident.
Never mixed diesel in a gas car, but did mix gas in a diesel. Mercedes recommends this on the 80s diesels to help it in cold weather. It also suggests mixing kerosene too if diesel isnt available.