What is Zeri's most reliable build when she mainly has to preserve useful distance in a long fight?

Zeri's most reliable build is the one that lets her keep the right distance while ramping damage. If your item order gives DPS but not enough freedom to kite, it is missing her real strength.

When should you truly adapt Zeri's build instead of simply pushing the standard mobile hypercarry route?

You should adapt Zeri's build when she no longer has time to let the fight stretch. If burst, poke, or overly direct engage cut off her ramp, you need to move up the defense or comfort that gives her that time back.

Which enchant should Zeri choose when the real issue is mainly staying present through multiple passes?

Quicksilver is often the most reliable choice when Zeri mainly risks losing her kite to one key crowd control effect. If the enchant lets her keep mobility at the key moment, it solves the right problem.