Will reacting to purple Kha'zix with a hidden Evelyn Entrancing cause his +2 might ability to fail?

No, reacting with a hidden Evelynn, Entrancing will not cause Kha'Zix, Mutating Horror's ability to fail, provided the condition ("an enemy unit is alone here") was met when Kha'Zix entered the battlefield.

Reasoning

1. Trigger Timing: Kha'Zix's ability is a triggered ability that checks the battlefield state at the moment he enters and gains a combat designation (Attacker or Defender). As supported by RiftJudge FAQ #9937 and #9849, because you are playing into a battlefield where the opponent's unit is "alone" at that exact moment, the condition is met and the ability is placed on the chain.
2. Chain Resolution (LIFO):
   • You play Kha'Zix (placing his trigger on the chain).
   • Your opponent reacts by playing Evelynn, Entrancing from face-down (placing her on the chain above Kha'Zix's trigger).
   • The chain resolves in Last In, First Out (LIFO) order.
   • Evelynn's ability resolves first. Even if she enters the battlefield and changes the number of units, it does not retroactively negate the trigger condition that was already satisfied when Kha'Zix's ability was placed on the chain.

According to Rule 359.3.f.4 and the logic established in RiftJudge FAQ #9937, because the trigger condition was met at the time the ability was placed on the chain, it does not matter that the battlefield state changes before the ability actually resolves.

This is my interpretation based on the standard rules for triggered abilities and the chain. No official FAQ exists for this exact card pair, but the interaction follows the established mechanics for Kha'Zix's "alone" trigger.