Will Obama over turn, Proposition 8 in CA. Through executive order ?

No, that's a legal matter that he would leave to the Judicial Branch of the government. What will happen is that some of these suddenly "de-married" couples will file lawsuits, which will be denied by the CA supreme court because they were overturned, and then the cases will go to the federal Supreme Court. They will overturn all of these "amendments" because there is no constitutional basis for them. Just like overturning the sodomy laws that existed in a few states a few years ago, this will go the same way.