The diesel nozzle is too big to fit into the petrol hole usually or has some sort of design that prevents it from entering easily.

Not where I live, they're the same size just different colors

Never underestimate the power of brute-force and stupidity. I know a couple friends who have filled their gas car with diesel, despite different sized nozzles.

the fraudster has gotta be pretty dedicated if they decided to memorise the card number instead of just using the card. i know my card number because i've used it so often.

You underestimate the dedication of someone who wants free money.

Sometimes if you're a fraudster, you don't have access to the whole card to steal it, but you have access to the number. Also, if you steal someone's credit card, they'll most likely notice it's missing. But people don't often check their balances, so if you were just using a number, you would be more likely get away with it for a while. People realized this, and that's why they don't let you pay just using a number anymore. In most places anyways.