Which Kha'Zix build stays the most reliable?

Kha'Zix's most reliable build is the one that protects the first real access timing without completely sacrificing the exit. A clean lethality base remains standard, then you adjust depending on whether the game mainly asks for more safety, better angles, or a stronger answer into grouped teams.

When should Kha'Zix's build be adapted?

You should adapt as soon as the enemy draft removes too many natural isolations or punishes the exit too hard. If the enemy team groups quickly, if burst stops you from replaying after entry, or if you are already behind, moving up Edge of Night, Guardian Angel, or an equivalent slot becomes more logical than forcing the ideal lethality curve.

Which enchant on Kha'Zix when the first reset is no longer guaranteed?

Stasis or a similar defensive slot becomes relevant when your real problem is no longer killing the target, but surviving the immediate answer. If the lobby closes your exit angle too quickly, protecting that second is worth more than a bit of extra damage.