What is Nami's most reliable build when she mainly has to control the tempo of the first and second trades?

Nami's most reliable build is the one that improves both her first trade and her ability to play again right after. If your item order helps neither her buffs nor her second re-entry, it is missing what wins on the champion.

When should you truly adapt Nami's build instead of simply pushing the standard enchanter route?

You should adapt as soon as the enemy draft stops you from playing the timing the build was meant to serve. If you need to survive longer, absorb more poke, or answer a specific engage pattern, the purchase order must move before you even think about the final six-slot build.

Which enchant should Nami choose when the real problem is mainly preserving a clean second re-entry?

Redemption often remains the most reliable choice when Nami mainly needs to extend an advantage already created by her first trade. If this enchant helps her team keep the tempo, it gives a lot of value.