Can Rebuke be played in response to Falling Star to return a target to hand, since Falling Star is a spell without Action or Reaction?

Ruling: No, you cannot play Rebuke in response to Falling Star. When a spell like Falling Star starts a chain, you can only use Reactions to respond to it, not Action spells.

Nuances:

• "Action" does not mean a higher spell speed; it means the card can be played as the first card in a chain when you get Focus during a showdown
• Action spells/abilities cannot be used to react to normal spells/abilities
• Gust can counter Falling Star if the targeted unit is 3 or lower Might and in the Battlefield
• If Gust removes all valid targets, Falling Star's reflexive triggers will force the caster to target their own units