Does elemental mastery stacks across different specialisms of the…
This is correct, so having mastery in Air, Dark and Water means you can cast the third level Invisibility (Air/Dark) for 1MP and the 5th level Steal Power (Air/Dark/Water) for 2MP.
Does elemental mastery stacks across different specialisms of the skill, reducing a spell cost by 1MP per skill for each relevant element used to cast a spell?