Wrong fuel in MGF?

Wow! 135 to 155 °C (275 to 311 °F). It wont go back to chrome, live with it or have it re-chromed. In the gun trade blueing is done to protect the metal from corrosion, it's a skill when done by using heat. Thickness of an exhaust system (quality) can also play a part in how easily it is effected by heat.

Use Peek metal polish. It will remove it.

Nothing to do with the fuel. Bluing happens. It's a fact of life. Most chrome bike exhausts blue quickly.

Bluing doesn't reverse automatically. It was also highly unlikely to have been caused by the wrong grade of fuel; that was likely coincidental.

Many metal polishes will remove the bluing, but a lot of sources on the web also claim that rubbing with white vinegar or lemon juice (both are just weak acids) will remove it. What definitely will work is polishing vigorously with toothpaste or household scouring powder.

All methods work by removing the surface passivation layer from either chrome or stainless steel. That layer is usually clear and invisible and forms from oxidation providing a tough protective layer. But whether it's clear, blue, or brown depends upon its thickness. Removing the layer with polishes (abrasion) or weak acids allows it to re-form at the correct thickness. The fix is not permanent: whenever those tips get very hot they will go blue (or brown) again. When they do just remove the bluing again.