How does Tideturner and Overzealous Fan work when I play Tideturner in the first priority step during a showdown and swap it with Overzealous Fan? Does the When I defend trigger still trigger and what would be the first possible point to play Tideturner as a defender?

To address your questions regarding the interaction between Tideturner and Overzealous Fan during a showdown, here is the breakdown based on the official rules and mechanics.

1. Does the "When I defend" trigger still trigger?

Yes. When you use Tideturner to swap a unit into the defender position during a showdown, that unit gains the "Defender" designation. According to Rule 376.4.e, "When I defend..." triggers occur when a unit gains the Defender designation for the first time during a combat.

Because Tideturner's ability moves the target unit into the combat, it will gain the Defender designation during the cleanup phase following the resolution of Tideturner's ability. Consequently, Overzealous Fan's "When I defend" ability will trigger at that time.

2. What is the first possible point to play Tideturner as a defender?

You can play Tideturner from Hidden as a reaction as soon as you have priority.

In a showdown, the first opportunity to play a reaction occurs after the Initial Chain is created (if one exists) or after the showdown begins (if no initial chain is created). If you are the defender, you gain priority to play reactions during the Initial Chain (if you have any) or during the first Open State of the showdown when you have priority.

Sequence of Events

When you play Tideturner from Hidden to swap it with Overzealous Fan:

1. Play Tideturner: You play Tideturner from Hidden. Playing Tideturner starts a chain which immediately resolves.
2. Trigger Ability: Tideturner's "When you play me" ability triggers and is placed on the chain.
3. Reaction Window: Players now have the opportunity to play reactions to Tideturner's triggered ability.
4. Resolution: The chain resolves. Tideturner swaps positions with Overzealous Fan.
5. Cleanup/Designation: After the swap, the game performs a cleanup. Overzealous Fan (now at the contested battlefield) gains the Defender designation.
6. Trigger: Overzealous Fan's "When I defend" ability triggers and is placed on the chain.

Summary of Mechanics

• Hidden Timing: Per Rule 737.6, a card that is Hidden gains Reaction while facedown and may be played any time a card with Reaction may be played.
• Entering Play: As noted in FAQ 8196, Tideturner enters the battlefield before its "When you play me" ability goes on the chain. You cannot react to the unit entering play, but you can react to the triggered ability that follows.
• Triggering: The "When I defend" trigger is not missed because the unit enters combat mid-showdown; it triggers during the cleanup phase after the unit gains the Defender designation (Rule 376.4.e.2).

Note: I could not fully verify this ruling against the retrieved rules.