What is Soraka's most reliable build when she mainly has to stop the opponent from closing fights?

The most reliable build on Soraka 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 Soraka's build instead of 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 Soraka choose when the real issue is mainly staying alive long enough to invert tempo?

Redemption often remains the most reliable choice when Soraka mainly needs to buy time for her team. If the real need is lasting a bit longer before the fight turns, this enchant gives a lot of value.