Whats stopping Killua from farming wishes and optioning ultimate power...

Because Killua loves Alluka and views her as a sister instead of a wish granting machine like everyone else in the Zoldyck family. If Killua starts abusing that power there's no telling what Allyka will do

He only gets some degree of immunity from consequences on the whim of an eldritch abomination from the dark continent who scares the shit out of the most notoriously deadly professional assassins alive.
And like a complete baka he decided to snap at Nanika in some weird attempt to get her to never appear again so Alluka could benefit from it. So already he's on extremely thin ice.
Also keep in mind the consequences didn't disappear. Nanika is just waiting for other victims like Tsubone to take the damage in Killula's place each time. She basicly decided to skip Killula and defer the horror for later e.g. the fingernails thing was paying for the computer which should have been Killula's price but got deferred to the next victim instead.

thats right

Though that said there's some distinct other asspull going on where if anyone wishes for healing Nanika never has bad demands for the next person tied to that wish.
Again though, this is all at the whim of a psychotic demon entity from outside the known world.

Power not properly understood but also she is just there to revive gon in the story

the stronger the wish the more aura it's released, so using them is just painting a giant target on your back and making yourself visible to the wrong people.
Besides going for ultimate power seems kinda abstract, since even Meruem was just simply nuked, being that powerful is not ultimate power, so even if being immortal as well could be an option asking all of these in the seek of power goes wildly against Killua's character.
And Illumi's needle obviously altered his memory of Alluka.

Dude, it was one giant ass pull to fix Gon's deux ex machina moment. You are thinking too much. HxH is arguablly the best shonen battle manga of all time but it has some of the worst asspull moments in the history of the genre. The entire CA arc was undermined by the fact that at any moment Killua could have used his sibling to kill off Meruem.

>why just remember her after
>her
It's a boy cross dressing. Killua is the only one in his family that refers to him as her.

>Meruem was just simply nuked
That's another plot hole. Mereum is introduced as someone who is so big-brained he can beat a chess master after learning the rules a few minutes earlier. Yet this genuis IQ guy overestimated his ability so greatly and was dumb enough to invade the world without looking into what humanity was capable of. Also, he millions of people working for him, not one of them read a history book or knew about nukes? This is shit writing.

Intelligence vs wisdom.
He was literally a newborn baby.

I dropped the manga for that bitch.
Togashi is a hack.

>CA arc
Do you mean KA?