If I attack into an Overzealous Fan with an Irresistible Faefolk planning to use an Ambush unit, does Overzealous Fan have to decide whether it'll kill itself on defend to move Faefolk back before I react with Ambush, and after it decides not to kill itself on defend trigger, can I still play Ambush?

To determine the timing of this interaction, we must look at the sequence of a combat showdown and how triggered abilities are added to the chain.

The Short Answer

No, the owner of Overzealous Fan cannot "wait" to decide whether to kill the Fan after seeing your Ambush play.

When Overzealous Fan triggers, it is placed on the chain immediately. The controller must decide at that moment whether to place the ability on the chain (if they choose "yes," they pay the cost of killing the Fan to place it there; if they choose "no," they do not). Once the ability is on the chain, players alternate priority to play reactions. If you use your Ambush unit as a reaction to that trigger, you are playing that unit while the Fan's ability is still waiting on the chain to resolve.

The Detailed Sequence

1. Attack Initiated: You move Irresistible Faefolk into a battlefield occupied by Overzealous Fan. The battlefield becomes contested, and you are the attacker.
2. Triggers: Both Irresistible Faefolk (When I move...) and Overzealous Fan (When I defend...) trigger.
   • Note: Because this is the Initial Chain of the showdown, these triggers are added to the chain simultaneously. If the active player (the attacker) adds their trigger first, the defending player adds theirs, or vice versa depending on who is designated the primary initiator of the chain.
3. Fan's Decision: When Overzealous Fan's "When I defend" ability is added to the chain, the controller must decide right then if they are using it.
   • If they say yes: They kill the Overzealous Fan immediately as a cost to place the ability on the chain (RiftJudge FAQ #8702).
   • If they say no: The ability does not go on the chain at all.
4. Reaction Window: Once the chain is established, it is a CLOSED state. Players can now play [REACTION] cards. This is when you can play your unit with Ambush (as Ambush allows you to play the unit as a reaction).
5. Resolution: Players continue to add reactions until both pass. Then, the chain resolves in reverse order (LIFO).

Key Takeaways

• The Cost is Upfront: The controller of the Fan cannot "wait and see" what you play before deciding to kill the Fan. The decision to kill the Fan happens at the moment the trigger is placed on the chain. If they pass on the trigger, they cannot trigger it later in that same showdown.
• Ambush Timing: You can play your Ambush unit as a reaction to Overzealous Fan's triggered ability on the chain.
• Showdown Persistence: Per RiftJudge FAQ #8798 and #9694, the showdown does not end if Overzealous Fan moves your unit or is killed. You retain the ability to play [ACTION] cards or additional [REACTION] cards as long as the showdown is active and you have focus.

This is my interpretation based on the mechanics of triggered abilities as costs and the defined showdown timing. No official FAQ exists for this specific combination of Irresistible Faefolk and Overzealous Fan.