This part feels clear for me but Im still not convinced on what to do when a character draws Visit the city when no such space exists on the game board. I see this as the same scenario as The take one life quest vs CoC.
You can always bang a Gong of War in Highlands or use other ways to summon a character from CoC... So there is always way to complete the Warlock quest even if the space you have to visit is terrianed .
But on the other hand if completing the Quest would be too difficult than their use become marginal. Strenght of Talisman still comes from the fact it is consider as a familly game and it can not be too complicate.