The ball that's too tight to the net: The setter must jump above the net and maintain the hand position and square the hips in order to make a quick set for the middle hitter. Feet must get to the ball before jumping for the ball, and the jump should be primarily vertical rather than horizontal.
The single-handed set on the net: Get the feet to the ball, square the hips, and pop the ball up with one hand for the middle hitter.
When the sitter can't get to the ball quickly enough: Drop down into a lower stance or come down to the knees. Turn the hips into the court and create a small set to the middle hitter.