Hi PikonParadox. Thank you for playing! (there are only hundreds of other submissions... yikes.)
I think an 'undo' mechanic has value - I'd need to rework the enemy and give them a proper AI to do this correctly: they don't respond to your actions today, they play a predetermined series of cards no matter what the player does. So without an AI rework, offering an undo gives the player insight into the next play of their opponent - which could be exploited. :|