What is Nocturne's most reliable build when he mainly has to engage without automatically trading the dive?

The most reliable build on Nocturne is the one that preserves the first spike while still keeping an escape valve. If your item order prevents you from playing the champion's first real window, the build may look strong on paper but it is not reliable.

When should you truly adapt Nocturne's build instead of simply pushing the standard assassin-bruiser route?

Loses value against tanky comps, heavy CC or strong peel where he cannot finish targets quickly.

Which enchant should Nocturne choose when the real issue is mainly living one second longer after ultimate?

stasis is often the most stable choice because it directly supports this game plan: maximize one-shot potential on engage while keeping enough survivability to secure the kill and reset map pressure.

Nocturne Wild Rift Best Build Guide (Patch 7.2a)

The build maximizes instant burst to capitalize on each ultimate while adding just enough safety to exit after the pick. Nocturne plays short decisive engages, not extended fights.