Swift

Swift's Tier II Alt Green says "*Move an enemy minion in radius up to 3 spaces to a space in radius. Next turn: The first time an enemy minion in radius is defeated, gain 1 coin.*" Swift can choose to move an enemy minion 0 spaces, but does she have to target an enemy minion in the first place to then move it 0 spaces? If there are no enemy minions in radius, is a mandatory step failed so the Active Effect is not activated? Swift's Tier III Alt Green says "*Move up to two enemy minions in radius, up to 3 spaces each, to spaces in radius. Next turn: The first two times an enemy minion in radius is defeated, gain 1 coin.*" In this case Swift can choose to move 0 minions, and if there are no enemy minions in radius then a mandatory step is not failed. Are these cards intended to work differently?

Follow the text as written. Higher tier card is stronger.

Was Swift's silver card changed in the update? Doesn't swap anymore? Or is it printing mistake?

This is not a rules question. Most cards were changed in the 2nd printing.

If swift repeats his red card with his gold card can he target gydion while invulnerability is on?

Why would it become both? You have Action 1. Then Action 2.

for Chilling Howl combined with Swift Justice - does their "max movement" become the limited 2 or if they defend with a boot of 3, they just don't move since they can't hit the max?

Max movement is their [Movement] value. Whisper modifies that by reducing it by 1.