One-handed set
 

If the libero pops the ball up with one open hand above her head, is this considered a set if done in front of the attack line? The rule doesn't say that the setting action has to be performed with two hands.

Would it make a difference if the contact is clearly made with only the heel of the hand, rather than the palm?

Set; either way

I don't know. I went home and looked up the role in the USA book. 19.3.1.4 uses the phrase "overhead finger pass". So, using the heel of the hand doesn't seem to fit that description. Even an open palm seems questionable. But I'm really not sure.

Example 1: Yes. It's overhand finger action
Example 2: No. No finger action.