If Killua later realized exactly how little Gon values himself, specifically pertaining to their relationship, how do you think he would approach it? (For example, it coming out that Gon clearly has no idea that he is as important to Killua as he is or maybe even bringing that "second place" comment back into play)

that depends greatly on the circumstances and on how much more self-aware killua himself has become by the time he has that realization. in a way, i think killua already knows that; he’s been witness to every single time gon showed his self-destructive near suicidal tendencies. being this close to a person that puts no value on their own life must be terrifying at best, and i do believe that this fear was part of why killua decided to separate from gon for the time being (the thought ‘if gon is so ready to toss away his life, then he might be just as inclined to sacrifice others’ must have crossed killua’s mind).

so far, killua has tried approaching this problem in the worst way possible - by ignoring it, his most involved response being “wow gon you are so selfish”. i can’t blame him: he has no relevant experience for dealing with suicidal adrenaline junkies (you kind of. have to go to school for that). the good thing is that killua has decided to have another person as his first priority. and i can bet that alluka won’t tolerate it for a long time, so killua will have to concentrate on himself and his issues eventually. 

best case scenario would be killua sitting gon down and talking openly about his self-worth issues and their relationship, but let’s be realistic, no one ever does that, unfortunately. i’m sure that by the time they meet up again, killua will still be a reticent emotional wreck, and as we know, the only time killua expresses his emotions is in front of any random person but gon, and in a decidedly. explosive fashion. i believe it would be rather in character for him to have a bit of a mental breakdown in front of someone else (hopefully a friend), and maybe get some useful advice or insight after that, the way it happened with palm, for example. hopefully, afterwards he would seek gon out and at least try to broach the subject. they definitely wouldn’t be able to cover everything in one conversation, but it would be enough to make a start on long painful work on gon’s issues and their relationship alike.