That Destiny and Mystique voted differently might be the key. With Destiny back, Mystique doesn't really offer the council any special strengths or skills or power. Destiny can see the future, she's necessary. Mystique is not. The council voted Hope in after Magneto leaving because of Hope's status as the mutant messiah and of her position in the 5. So I think in order to give Selene a seat they are going to vote Mystique off, and obviously Destiny knows this, but didn't tell Raven because this needs to happen for the good of the future. Plus Mystique can be moved to a title where see her in the field. Maybe she joins XMen Red.