I like that part of mining leaving China as well. What I don't like is a +50% less secure bitcoin network precisely for the reasons LoyceV mentioned in his post above. Although I don't find the threat of a 51% attack that realistic because it's almost impossible that all the missing hashing power could end up in the hands of the same individual or a malicious group who are going to use it for such a purpose. It's even less realistic that those who are still providing their computational power to the network do the same thing. A combination of 1 and 2 makes more sense, but I doubt it.     

Sure, but that is going to take time. It's not a one-click process. If the miners decide to sell their equipment, it's gonna take weeks to pack up, ship, and connect and configure at the new destination. If they decide to leave China to mine somewhere abroad, it might take even longer. Finding a new location, ensuring you understand and comply to local laws and regulations, finding an electricity provider, filling out the paperwork, setting up your farm...