What is Alistar's most reliable build when he mainly has to survive the entry and then replay control?

The most reliable build on Alistar 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 Alistar's build instead of simply pushing the standard engage tank 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 Alistar choose when the real issue is mainly holding on after the first combo?

Locket often remains the most reliable choice when Alistar mainly needs to hold after entry. If the real problem is the immediate answer onto him or his carry, this enchant gives more value than a more aggressive choice.