As this is one of Compture's primary domains it would help if we had a clear idea of what the Change Shape turning substitute involves. Presumably it is similar to the Druid ability?
I think the mischief ability should affect others, like the Gremlin ability, but should not be active all the time. How about a pool of points, equal to level, that you can use to reduce another's rolls. So you target an enemy with a spell and say, before they roll their save, I will take 3 points off their roll. Same if being attacked etc.
