If I played Kha'Zix mutating horror as a reaction on a chain, and then later on I use gust twice on the same chain to remove 2 out of the 3 enemy units back to their hand, leaving only 1 of their unit on the battlefield now, does my Kha'Zix mutating horror get its effect of an enemy defending alone, thus getting +2 might?

No, your Kha'Zix, Mutating Horror will not receive the +2 Might bonus in this scenario.

Reasoning

According to the rules for triggered abilities and the specific "alone" condition on Kha'Zix, Mutating Horror:

1. Trigger Timing: Kha'Zix’s ability is a triggered ability that reads: "When I attack or defend, if an enemy unit is alone here...". As established in RiftJudge FAQ #9849, #10176, and #10198, the condition "if an enemy unit is alone here" is evaluated at the exact moment the unit gains the "Attacker" or "Defender" combat designation.
2. Condition Evaluation: When you played Kha'Zix via [Ambush] as a [Reaction], he entered the battlefield and gained the combat designation. At that specific moment, your opponent had three units at the battlefield. Because there were other units present, the condition "an enemy unit is alone here" was not met.
3. Result: Because the condition failed at the moment of the trigger, the ability did not trigger and was never placed on the chain.
4. Gust Timing: Playing Gust later on the chain does not retroactively change the state of the battlefield at the time Kha'Zix entered. Since the trigger condition was false at the required moment, changing the battlefield state afterwards does not cause the ability to trigger after the fact.

As stated in RiftJudge FAQ #10172, if the condition is not met at the moment the unit enters and gains the combat designation, the opportunity to trigger is missed and cannot be reclaimed by changing the board state later in the same chain sequence.