Should Norra always play full poke even when the game mainly threatens her right to stay at range?

Not automatically. A full poke build remains ideal as long as Norra keeps a real working distance and can convert her first hits into durable pressure. But as soon as dive or burst remove that right to cast, buying more safety often produces more real value than a simple increase in theoretical AP.

Horizon Focus or Infinity Orb second depending on whether Norra mainly wants to chip or finish faster?

Horizon Focus comes first when you mainly want to win before the real commit by making every long-range hit much more expensive and easier for your team to convert. Infinity Orb becomes better when the game is decided more by quickly executing already chipped targets than by methodically wearing down approaches.

Is Stasis mandatory on Norra when the real issue is keeping her zone open long enough?

No, but Stasis very often becomes the best enchant as soon as real entry threat exists, because it protects the exact moment when Norra only needs a few extra seconds to break enemy tempo. If nobody can truly shut her down, a more aggressive enchant can still be defensible.

Norra Wild Rift Best Build Guide (Patch 7.2a)

Norra does not win by entering melee, but by controlling distance and making every enemy approach costly. Her AP poke build increases the value of every long-range hit and strengthens her ability to shape the fight before it truly starts.