I've just done some big clean-up and improvements to this activity. About your first request, that won't really change as the way it works is to move an item by clicking on its destination slot, which in my opinion is good. But the visual improvements should make it more obvious (before the ListView was not clipped, so it could show an extra item not clickable on the bottom, which was indeed confusing).

About the OK button, I would not like to add one by default, as the instant feedback is part of the good experience when using this activity. Though maybe we could add a "test mode" option which would not give any visual feedback and require to press an OK button to validate the answer.