Years ago I would sometimes travel about Colorado w/a relative in his 4-seater airplane, and my job was to test the plane’s gas tanks for water intrusion prior to each take off. There was a little valve at the bottom of each wing, I’d press it, and a little liquid would come out. As long as it smelled like gas, good to go. See if you can come up w/a way to do that with your water bottle sample so you are certain what those two layers actually are. There’s definitely some kind of major contamination there. It’s possible that same layering effect was occurring in the station’s holding tank, and you just happened to get the brunt of it. The cars who were filled afterward didn’t, b/c your car got the entire layer of water.
Do you think you may have been on the end of a trick or a practical joke? Could someone have poured water into your tank as a prank? That’s another idea.
A final idea, I don’t know what a mix of diesel and gasoline looks like, but if the diesel pump was used, this symptom could happen. While it’s more or less impossible to fill a diesel car w/gasoline b/c the gasoline nozzle is bigger diameter than a diesel car’s fuel fill hole, it’s definitely possible to fill a gasoline car w/diesel. A neighbor of mine had that exact problem last summer. Engine started to chug and soon came to a halt a few blocks from the gas station.
If the testing shows that indeed it was water, given the evidence presented by your shop, I think you have a good chance of being reimbursed for your trouble by the gas station owner. It may require some persistence on your part though. $540 seems reasonable for that.