Sleeve Aces has exhaust printed on it but does in fact not exhaust.

You are indeed correct! Not exactly sure how we missed that in testing, but good catch!

Is thespian form supposed to make the flourish that shuffles cards back make duplicates of cards?

no. That's a mistake on my part. Will be fixed in next update.

Okay so I played a run with multiple columbify and still have no idea how the card is supposed to work. Most enemies after becoming a byrd become stunned but untargetable the next turn regardless of if the byrd was downed or not (???), several enemies get lost in their attack patterns, donu will for example attack again afterwards, time eater forgets to cleanse despite starting the first turn he gets to act below half, byrd gets buffed by philostone.

The card is incredibly confusing and unintuitive and I have no idea how many of these are intentional.

It seems like Vanishing Defend is not tagged as BASIC_DEFEND (while Vanishing Strike is).