What is Sona's most reliable build when she mainly has to reach her scaling without losing control of tempo?

The most reliable build on Sona 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 Sona's build instead of simply pushing the standard scaling 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 Sona choose when the real issue is mainly replaying more small winning sequences?

redemption is often the most stable choice because it directly supports this game plan: maximize spell rotations to generate healing, shielding and constant poke in extended fights.