What is Samira's most reliable build when she mainly has to survive the entry and then convert resets?

Samira's most reliable build is the one that lets her go in, survive just long enough, and convert her first reset. If your item order only protects damage, you leave the hardest part of her job unanswered.

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

You should adapt Samira's build when she can no longer reach her reset under good conditions. If enemy control, burst, or frontline stop her before her real entry timing, you need to move up survivability or the right penetration.

Which enchant should Samira choose when the real issue is mainly surviving the second that leads to a reset?

Stasis is often the most reliable choice when Samira can find the entry but risks dying before finishing the sequence. If the enchant buys the second she needs to ult or reset, it is doing its job.