Re: How did Kite come back to life as an ant ? ( or better yet, at all )
For me it's easy: When in the verge of death, Nen activates and finds a new hostage and move the soul over there. Probably with a condition and some kind of particular filter in order to go to the nearest "mother".
If creating a sword that can cut through everything is beyond the limit of Aura, then resurrecting a dead person is definitely also beyond the limit of Aura.
ACTUALLY...It's not what Mizuken meant.
Rebirth is possible, you could see that souls were dealt somehow at Greed Island. The sword can cut trough everything IS impossible because you can never KNOW everything. So unexpected things are what makes that sword impossible, not the feat of having an incredible sharp sword. In the same way, you can heal something enough to make a limb again, heal faster than human possible and even use your Nen to get away from death.
Aura cannot even reverse aging in a practical way. There's an item from GI that can reverse aging, but it is clearly not usable in any practical way because it'd be pretty insane for Netero to strap a bomb on himself instead of just reverse his aging to a point where he'd be twice as strong (he only has 50% of his peak power). Okay maybe he would need to strap on a bomb on himself even at his full strength but certainly it's not going to hurt if he returned to his peak before fighting a battle that was supposed to determine the fate of humanity. If you can cheat death the world of HXH would basically look a lot like Final Fantasy X's Spira where every person who didn't want to die sticks around just because they can.
It could if your technique is only dedicated to that. For examples, look at Biscuit. Netero focused his technique in something else which was HIS style to fight and what interested him the most.
And not anyone can, only people who are interested into it and make the needed vows and restrictions which I'm sure Kaito technique has.
By the way while there's a concept of a 'spirit' (Pitou says he can't bring Kite back because his spirit is no longer around), we don't see the Zodiac consulting the spirit of Netero to see if Beyond Netero was really his son, so even if you go that path at best you might go with something like "Kite's spirit will always be with you, even though you can't possibly sense him".
Which is a good way to describe that it was somewhere else. :/ Probably, in another body.
And I doubt they do that, actually, since the costs of that technique could be dangerous and unneeded. Also, we know little about those kind of techniques, but I'm sure there is someone in the HxH world purely dedicated to that.
After all you have a person who can see 100% in the future. Which is human impossible.
PS: Avoid offtopic and Star Wars chit-chat.