It would have to be a bigger operation than just that one dam. They would have to clear out five more dams downstream, making six dams in total ... before any water would ever get to Baghdad.
Actually, most of the other dams are not under Turkish control IIRC so they might be used to moderate downstream flooding. That said, several are in Syria, so that might be a problem.
Not necessarily. If the top dam is full to the brim and is made to catastrophically fail, the resulting rogue wave of millions of gallons of water traveling down stream, could cause the downstream dams to similarly fail (especially if they were sabatogued to be susceptible to such a mass impact of water hitting them. A domino effect of destruction through Syria and Iraq.
Wouldn't be the first time that ME terrorists tried to recreate fictional destruction in reality.