What is Sivir's most reliable build when she mainly has to impose a front-to-back faster than the opponent?

Sivir's most reliable build is the one that lets her hold a front-to-back without losing her auto rhythm. If you add damage but can no longer play the full duration of the fight, the build becomes less reliable.

When should you truly adapt Sivir's build instead of simply pushing standard crit?

You should adapt Sivir's build when she no longer has enough safety to stay in firing range. If poke, dive, or an overly heavy frontline stop her from keeping autos useful, you should fix that before pushing standard crit.

Which enchant should Sivir choose when the real problem is mainly keeping the right to auto through the whole sequence?

Quicksilver is often the most reliable choice when Sivir mainly loses value by being stopped during her auto sequence. If the enchant simply lets her keep playing the fight, it is often correct.