Calculating Germany's third city could produce a few different answers, depending on whether you treat the Ruhrgebiet (the multi-city Ruhr Valley area), and the Köln/Düsseldorf urban area as single "cities" or not. We have not, hence Hamburg.
By some measures it is! Populations have been calculated for this quiz by urban area; using the city-administrative-boundary method you'll have seen in Wikipedia, the third ranked city is Munich.
Yes, but I was pretty confused when Venezuela didn't work at first. Of course, I then tried Spain and tried Venezuela again. I wonder if there's any way to make both work for either spot.