Can a unit defending be targeted by Existential? If so, if that unit is already stunned, could you send it back to the opponent's hand?

To answer your question, we must look at the targeting requirements for Existential Dread.

Can a defending unit be targeted?

No. According to the card text of Existential Dread, it specifically targets an "attacking enemy unit."

In a Showdown, combat designations are assigned to the units involved: the unit initiating the conflict is the "Attacker," and the unit(s) it is facing are the "Defenders." Because Existential Dread requires the target to be an "attacking" unit, it cannot target a unit that has been given the "Defender" designation.

Interaction with a Stunned Unit

Since you cannot target a defending unit with this card, the second part of your question does not apply in that context. However, for clarity on how the card interacts with an Attacker that is already stunned:

If you were to target an attacking unit that is already stunned, Existential Dread would indeed return it to its owner's hand. As stated on the card: "Stun an attacking enemy unit. If it's already stunned, return it to its owner's hand instead."

Summary:

• Defending units: Cannot be targeted by Existential Dread.
• Attacking units: Can be targeted. If the attacking unit is already stunned, the effect replaces the stun with recalling the unit to its owner's hand.