Fireball Rune Trap Questions

When reading through the hazards, I came across the fireball rune. After reading its entry, I walked away with some confusions on the rules for it.

1) There is a thievery entry for it, allowing it to be disabled upon a successful thievery skill check. However the trap detonates as soon as you enter it’s 20ft sphere of detection. From my understanding, you need to be adjacent to the hazard to attempt to disable it. Is the intention that no one can actually get close enough to the rune without setting it off.

2)The fireball rune is triggered by a “living creature” entering its area. If the above statement regarding there being no way to disable it with thievery without setting it off is true, is this supposed to be something only undead pcs can disable with thievery? Is this trap just a hard counter to a non magical, living rogue?

3)Also as part of the “living creature” clause in its trigger entry, what exactly counts? Does something as small as a fly or spider entering the detection sphere set the trap off? Additionally, how does someone even set this trap up without killing themselves by being in the trigger range? Wouldn’t it go off the second they finish putting the rune down?

I have included a picture of the traps stat block for reference.