Placing the flags where the country is - fantastic idea! I don't know if it's doable but I suggest having a non-white background (light grey?) otherwise the white in the flags gets confused
Nice quiz! Also, I notice these are almost all very good (Niger and Latvia are probably the worst, but still very good) flags. Also, Saint Vincent's flag has the symbol of a V (for, well, Saint Vincent).
If you consider St. Lucia and Guyana's triangle colours separately (which you probably should?) they are also pretty irregular (although at least symmetric)