What is Lulu's most reliable build when she mainly has to amplify the right carry without losing defensive control?

Lulu's most reliable build is the one that helps the right carry stay playable while keeping enough defensive tools around that target. If your item order improves neither survival nor impact for the main carrier, it is missing its real target.

When should you truly adapt Lulu'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 Lulu choose when the real issue is mainly punishing over-optimistic focus?

Redemption often remains the most reliable choice when Lulu mainly needs to buy time for her carry. If this enchant lets the main ally stay useful for longer, it gives a lot of value.