So, currently, people enemied to a Prime area are open to PK. However, when a person not enemied to the area (let's call him PersonA) attacks a person enemied to the area (PersonB), PersonB cannot fight back without being blocked by Avenger. Further, if PersonB succeeds in killing PersonA, PersonB gets status even though PersonA was the first one to attack. PersonA suffers no such drawbacks.

Can it be changed so that if PersonA initiates the fight with PersonB, the latter is able to fight back without drawing the attention of the Avenger?

It's not perfect the way it is, but it's better than this, I think.

It's a bit of a limiter on Prime-raids; otherwise a Druid could plop a demesne in Serenwilde and camp out for hours on end. Anyone who actually attacks can be split apart and killed by the Druid (or Druid & friends)... even breaking the demesne is considered by the game to be an attack.

It's different in non-org territory but they've yet to make a distinction between enemy organizational territory and NPC enemy territory (that 2 million essence loss in enemy NPC territory is kind of excessive, for instance, as it was put in as a limiter on org-raiding).

If at some point they split it up, then that's a different matter. As-is, just have to be aware that that's how the Avenger works; if you're in enemy territory and someone attacks you, it's just like you typed DECLARE PERSON on them.