Actually the Ice King found the crown when humans lived like we do now and became mad because of the crown. The crown made him hear voices, turned him blue with white hair, lowered his body temperature and gave him special powers. His fiance left him as he had "changed". I believe he worked as an archeologist and found the crown on one of these trips. This might be the reason why he survived the mushroom war. Princess Bubble Gum also wears a crown, so they might have been working together and found the crowns at the same time. Princess Bubble Gum doesn't really have any special powers though, she is very smart, but that isn't really anything like being able to control snow and ice. But both look human have turned a different color and are more similar to their "element" as the princess is made of candy and the Ice KIng is ice cold. Just my theory, but I agree with you on The Mushroom War actually being a nuclear one. I believe the episode where you find out about the Ice Kings past is the christmas special :)