How likely do you think it is that Killua and Gon are keeping in touch?
i can imagine it being either way, but personally i think they do keep in touch, sending each other e-mails and pictures fairly often.
however, it’s all rather superfluous, as it is bound to be between two friends that had a falling out and never talked about it. both of them probably feel the tension seeping through the lines of their letters (gon knows that killua is angry with him but refuses to talk about it with gon, and killua is an anxious mess).
i suppose by the time they meet up again they won’t have resolved the conflict, but rather buried it under pleasantries and fake-casual conversations, which will make the whole ordeal a bit more difficult than it could have been.
If Illumi pulled his head out of his delusional ass and realized how important Gon is to Killua, do you think he would try targeting Gon on Whale Island in order to get to Killua and Alluka? And on that note, if Illumi did target Gon, how do you think Killua would react?
frankly speaking, i don’t think that illumi is the kind of villain that would hold someone hostage to get to killua. illumi is not and has never been a person that knows how healthy relationships work, what love feels like, and he plain cannot acknowledge how important gon is to killua even in the face of the most obvious of arguments — killua trying to save gon’s life and facing off his family in order to do so. holding gon hostage wouldn’t even cross his mind, because to him, he is the person killua loves the most: no on else would compare.
but if illumi did capture gon, i imagine killua would have a mental breakdown trying to figure out what he was supposed to do with alluka while he rescued gon (which would not even be a question to him). this would be the most difficult decision he would have to make.