I'd leave those two nuts alone. They set the rod length and preload, so if you do move them mark the position with a dab of paint first. The rod usually feels solid by hand because of that preload, so on its own it doesn't mean anything is seized. To check the flap you can take off the clip holding the rod end on the wastegate arm and slide it off the pin. The arm should then move freely through its travel by hand. If it's stiff or gritty the flap is your problem rather than the actuator.