However, there is a distinction between the power of Preservation and Leras, the entity which controlled the power. When Leras died, the mists remained. Likewise, when Leras created the prison of the Well of Ascension, he was a weak, shadow of his consciousness. The mists and the mist spirit acted separately in this regard. If the mist spirit could still control the mists fully and completely, there would be no reason for him to keep using the Deepness to make people Snap.
In MB2, the mist spirit is trying to prevent Vin from releasing Ruin at the Well. Well, the killing-mists is a large factor of Vin going to the Well of Ascension. If Leras/mist spirit wanted to really prevent the catastrophe, then he would have tried to stop the mists from making people Snap, because that would prevent the "Deepness". Since the mist spirit did not do that, I can only assume that is because the mist spirit did not have enough strength to control the mists. This shows that there is a clear distinction between the mist spirit and the mist, even if they are both part of Preservation. They are not interchangeable.