What is Bard's main role when he is not only playing lane but the whole useful map?

The most reliable build on Bard 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.

Why prioritize mobility so heavily on Bard when he also has to remain useful in the real teamfight?

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 Bard choose when the goal is to preserve picks, saves, and objective tempo at once?

protobelt is often the most stable choice because it directly supports this game plan: this build maximizes bard’s ability to appear unexpectedly, create instant advantages, then disengage before enemy response.

Bard Wild Rift Best Build Guide (Patch 7.2a)

Bard does not win through stats but through imbalance. This build allows him to arrive first, create immediate advantage, and leave before the enemy can respond, turning movement into real pressure.