In reaction to Overzealous Fan dying, can I ambush with Kha'Zix and get the +2 for defending alone?

Yes, you can ambush with Kha'Zix, Mutating Horror in reaction to Overzealous Fan's ability, and he will successfully receive the +2 Might bonus and 2 XP.

Chain Sequence and Resolution

1. Overzealous Fan Triggers: When Overzealous Fan is designated as a defender, its ability triggers. Its controller decides to use the ability and pays the cost (killing Overzealous Fan). The ability is placed on the chain as a pending item.
2. Reaction Window: The chain is now open for you to play Kha'Zix, Mutating Horror as a [Reaction] using the Ambush keyword.
3. Finalization: You play Kha'Zix, Mutating Horror. The unit enters the battlefield and gains the "Defender" combat designation.
4. Kha'Zix Triggers: At the exact moment Kha'Zix gains the "Defender" designation, the game evaluates his trigger condition ("if an enemy unit is alone here"). Because Overzealous Fan is no longer present, the enemy battlefield state is considered "alone."
5. Trigger Placed: Kha'Zix's ability is placed on the chain at the top, above the pending Overzealous Fan ability.
6. Resolution (LIFO - Last-In, First-Out):
   • Kha'Zix's ability resolves first. Because the "alone" condition was satisfied at the moment he gained the combat designation, he gains +2 Might and 2 XP.
   • Next, the Overzealous Fan's ability attempts to resolve.

Key Clarifications

• Condition Timing: As confirmed by RiftJudge FAQs #10198, #10735, and #11284, the "alone" condition for Kha'Zix is checked at the exact moment he enters the battlefield and gains the combat designation. The game does not re-evaluate this condition upon resolution.
• Chain Placement: By playing Kha'Zix as a reaction to the Overzealous Fan ability, Kha'Zix enters play and his trigger is added to the chain above the Fan's ability, ensuring his ability resolves first.

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