I know QT, which abounds in KS, has a fuel guarantee. If bad fuel causes a problem with your car, they will pay. It stems from a time SEVERAL years ago when they had a BIG fuel contamination issue. Their paying may be due to an agreement they have with their fuel supplier(s), which indeed caused the problem.
Back in the olden days, many farm tractors were run on what they referred to as petroleum distillate, or distillate for short. It was very close to kerosene or diesel. What the farmers did was start their tractors on gasoline, and run them until they were warmed up. Then they switched over to the cheaper distillate. We had a very old plow tractor when I was a kid that still had the little gasoline tank and valve, but I don't think my dad ever tried running it on anything other than gas. This is why your car made it home OK. The engine ran fine as long as it was warm, but would not run at all well when it was cool.
I'd be surprised if the diesel mix really hurt anything, but you might want to get your favorite shop to take a second look. After all, if the supplier paid up once, they should pay again if you still have a problem.