for question 7, your senario Z X Y T U S W can't happen. Y cannot be third. With this senario discounted, at the end of the line, it's always going to be T S or S T, hence they can't both be red.
Adn for the first question, # 6, its the same reasoning. Y can't be earlier than fourth. It must be R G R then Y, since two red before Y. You made the inference, but failed to apply it.
I wouldn't be surprised if that was your confusion for question 11, as well.