If I move my Irresistable Faefolk to a battlefield with a Scuttle Crab and target a second Scuttle Crab in base, then react to the Faefolk trigger with an Ambushed Kha'Zix, does he get the +2 trigger?

Yes, your Kha’Zix, Mutating Horror will trigger and receive the +2 Might bonus and gain 2 XP.

Reasoning

According to RiftJudge FAQs #10198, #10735, and #10934, the condition "if an enemy unit is alone here" for Kha’Zix’s triggered ability is evaluated at the exact moment he gains the "Attacker" or "Defender" combat designation.

1. Move & Trigger: When you move your Irresistible Faefolk to the battlefield, it applies the "Contested" status. You are the Attacker.
2. Kha’Zix Gains Designation: As your Kha’Zix (played via Ambush) enters the battlefield, he gains the "Attacker" designation. At this exact moment, the game checks the battlefield state: "Is an enemy unit alone here?"
   • Since your opponent currently has only one unit (the Scuttle Crab) present at that battlefield, the condition is satisfied.
3. Chain Placement: Kha’Zix’s triggered ability is placed on the chain.
4. Reaction: Your play of Kha’Zix (via Ambush) and any subsequent reactions happen after the trigger condition has already been checked and validated.
5. Resolution: When the trigger resolves, the state of the battlefield (even if the second Scuttle Crab is moved there by the Irresistible Faefolk trigger) does not retroactively negate the trigger.

Sequence of Events

1. Move: You move Irresistible Faefolk to the battlefield.
2. Trigger: Irresistible Faefolk's ability triggers. It is placed on the chain.
3. Reaction: You play Kha’Zix, Mutating Horror using [Ambush].
4. Trigger: Kha’Zix gains the "Attacker" designation upon entering. His "alone" condition is checked and found to be True. His ability is placed on the chain.
5. Resolution (LIFO):
   • Kha’Zix trigger resolves first (granting +2 Might and 2 XP).
   • Irresistible Faefolk trigger resolves (moving the second Scuttle Crab to the battlefield).

The condition check for Kha'Zix is not re-evaluated when his ability resolves; as long as the battlefield was "alone" at the moment he gained his combat designation, he successfully gains his bonus.