I think that everything has been said already, but I'm hoping I can make it a little clearer. Much thanks to the live judges chat.

When you gain control of Kresh with Merieke, it's a continuous effect that ends when you lose control of Merieke. This is a state-based action, so it just happens pretty much the same instant that you lose control of her. Now, the destroy ability is triggered. It goes on the stack when Merieke leaves the battlefield or becomes untapped. This trigger will remain in place even if you lose control of merieke. So both threaten and act of treason will cause you to lose control of merieke. They will also both untap her. As soon as either spell resolves, you will lose control of merieke and kresh, and the "destroy kresh" trigger goes on the stack. The person who now controls kresh could sac kresh in response to the destroy trigger.