I just read that the character of Mellie has been added as one of the main full time characters; therefore, I don't think she was behind the shooting. I think it has to be someone dispensable to the program. I am still going with the VP. When she was Acting President, she was too willing to give up the supreme court nomination (which was the most important thing to her) to catch the shooter. I think she knew the name she would be given was going to be Huck because she helped set him up. She thought they would arrest him and think it was all solved and not look any further. Just my take on the story. Of course, a Hollywood writer I'm not.