What is Kalista's most reliable build when she mainly has to win through spacing and support synergy?

The most reliable build on Kalista 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 Kalista's build instead of simply pushing her aggressive ADC profile?

You should adapt Kalista's build as soon as the game no longer rewards raw aggression alone, but mostly the quality of spacing, the bound support, and execution length. If you read that shift poorly, you end up with a Kalista that still hops a lot but converts too few of her windows into concrete advantage.

Which enchant should Kalista choose when the real problem is mainly keeping the right to kite?

quicksilver is often the most stable choice because it directly supports this game plan: maintain constant pressure while staying untouchable through kiting.