Which Leona build stays the most reliable?

Leona's most reliable build is the one that secures access first, then survival right after impact. A coherent tank base is worth more than an early mix of utility or damage, because her real value comes from the quality of the engage that the team can follow.

When should Leona's build be adapted?

You should adapt the build if Leona reaches the fight too weakened, or if she dies too quickly right after engaging. If poke wears her down before entry, if burst cuts her all-in immediately, or if the game is already going badly, moving up a defensive slot pays more than an overly ambitious standard order.

Which enchant on Leona when the real issue is mainly surviving after entry?

Locket often remains the most reliable choice when your main job is keeping the engage profitable for the whole team. If the main problem is the immediate retaliation, this enchant gives more value than a purely aggressive option.