Is the iron on Earth's crust a leftover of the Iron catastrophe, or it was brought back by volcanoes?
Not all the iron sank to the core. Have a look at my answer to a previous question:

https://earthscience.stackexchange.com/a/7076/725

Your question is similar, but coming the other way. You're asking - why is there iron in the mantle/crust?

The answer is that there was enough oxygen left in the Earth after oxidising the other lithophile elements (Si, Mg, Al, Ca, etc) to oxidise some of the iron. And when iron is oxidised - it is not a metal anymore, but instead incorporated into rock forming minerals like olivine and pyroxenes.