Maybe I'm missing something here but it seems to me that, if you have both Dominance and Conquest victory conditions on then you will never achieve a Conquest victory since a Dominance victory only requires that you control 70% of the world, whereas a Conquest victory requires that you take over the whole world. Am I understanding this correctly?
Not quite. A domination victory depends on the amount of tiles in your culture radius and the population of the cities you control. If you play a scorched earth campaign and raze all the cities you capture from the AI then you aren't controlling the tiles and hence will never receive a domination victory.