What are the rules of nanika?
This theory says that Gon couldn’t use Nen because his body is unconsciously stopping him from experiencing the pain Gon went through beforehand. His Nen would not be fine because Alluka would “heal” Gon’s aura nodes which are needed open to be able to use Nen freely.
Gon apparently lost his nen after his fight with Neferpitou. The reason why that happened, according to his friends, was that the emotional trauma he faced when the reality of Kite’s death sent him so over the edge, that he created a temporal contract with his Nen in exchange for the death of those who wronged him.
Though he’s living and breathing, Gon is now no longer able to use Nen and must either navigate a new path to become a Hunter or find some way to get his powers back.
Killua does not die, He comes close to death in ep 100, at the end of the series Killua and Gon part ways so Killua can take care of Alluka, Every body who was supposed to die has died.
Does killua really die?
The Final Rule is that people who Something(Nanika) love can give her commands without any consequence on themselves or others. This ability is not borderline OP. It is totally OP. It would make sense that Illumi would do anything to access such power, using Killua as a means of indirectly controlling Nanika.
There are also extra rules for this Alluka/Nanika ability: