2004 Toyota Corolla Diesel in gas engine

Engine Mechanical problem
2004 Toyota Corolla 4 cyl Front Wheel Drive Automatic 76k miles

There was only about 2 gallons of gasoline in the tank when I accidentally put 5 gallons of diesel in the tank. I didn't realize what I had done until about 3 miles down the highway (going about 45-50 mph) when the car just stopped accelerating, but the engine didn't die - I turned the car off. So, I towed it to a shop. They went through two sets of spark plugs and quite a few hours of work.$600 dollars later, I took it home, and the check engine light came on for the first time after driving on the freeway for about 20 minutes. So, I took it to them again and they said it is my catalitic converter. They're not sure if it's bad, or just plugged. They said the part would be 600 dollars.I know, it contains platinum. So, I have three questions: 1.) If this part just helps to break down hydrocarbons and carbon monixide to cut down on emissions, will the car suffer AT ALL if we just remove it? 2.) If the converter is said to be plugged, can it easily be cleaned? (What would be reasonable for a shop to charge?) 3.) If the converter's melted, could that be attributed to the initial 3 mile drive directly after putting diesel in it? Or could the shop have caused it by not draining the diesel enough before running it? Thank you so much for your time!