Does Carnivorous Snapvine trigger a showdown that would allow playing action cards?

Ruling: No, Carnivorous Snapvine is a triggered ability that opens a chain where you can only play Reactions, not action cards. Showdowns only occur when a unit moves to a battlefield.

Nuances:

• If the targeted unit is buffed in response (e.g., a 6/6 unit buffed by +1 against 6 damage), it can survive with damage marked on it
• Units heal damage marked on them after combat and at the end of turn