Q: Can I keep Superhuman Strength in play if the attacked enemy is already stunned?

A: No, you must discard Superhuman Strength after She-Hulk makes a basic attack regardless of whether or not the villain is already stunned. -(Developer Ruling)

Q: Does Superhuman Strength discard after only using a basic attack or any Attack event?

A: You can play an Attack event to trigger Superhuman Strength and stun the attacked enemy. While it can create a situation where you miss out on the +2 ATK, that is ultimately the player’s choice. -(Developer Ruling, Hall of Heroes)

As I understand first FAQ is no longer correct via "1.5" Rules Change. In a She-Hulk "Jennifer Walters, Rules Lawyer" deck there was a text "Forced; article 1: “If a forced ability requires one or more targets to resolve and it has no valid targets, it does not initiate.” Furthermore, in subsection A: “Any costs that would have been paid to initiate the ability are not paid.” The cost in question: discarding Superhuman Strength. So now we can keep the card in play provided we don't have a valid target for its effect." that implied, that if target would be dead because of attack (so there would be no valid target to stun during responce timing) or if the villain would have a Stalwart keyword (so they couldn't get stun counters) or if they would just already has stun counter (and without Steady they couldn't get second one), then Superhuman Strength would NOT be discarded (and you could do some crazy stuff this way, gaining two +2 ATK boosts)

This card stays in play until you use an attack action with She-Hulk. At the moment, I think the only card that takes advantage of this is Counter-Punch (would do 5 damage and not discard Superhuman Strength).