@spiraling-back-to I’m gonna assume you’re asking your question in good faith so I will try my best to answer, even though my area of expertise as a meta writer is not Yato and Yukine:
Why does Yukine suddenly warm up to Yato after the ablution?
I’m not quite sure why you’re saying this was never touched on in the manga because it certainly was, even if it wasn’t outright said “I changed my mind because [reasons].”
This is the pivotal moment of their relationship, where Yukine goes from being angry and even somewhat resentful of Yato:
The size of the panels and the emphasis on Yato’s words, especially with the second page, convey to the readers the sheer importance of these words: I gave you the name of a human, so live like one. Live.
The whole point of the first arc was that Yukine was this 14 year old who is mentally forever stuck in the mindset of a 14 year old, who’s angry, who’s acting out, who’s stealing and behaving badly… in hindsight, we know that this is partly also because of how Yukine was treated by his father in his previous life - childhood abuse causes all sorts of behavioral problems and an impaired ability to form lasting and meaningful relationships - but the reason we see in this arc itself is that Yukine is acting out because he realizes that the things he wants - namely, a normal human life - is something he can no longer have. He is dead, stuck with a homeless bum god, and has no purpose in his life.
But Yato gives him that purpose. He tells him “you have a person’s name, you can still live even though you’re dead, you have a place here.” Not only that, Yato backs up his commitment to Yukine with his actions: the fact that even though Yukine has hurt him so badly, stung him almost to the point of death, he still will not give up on this kid that he has taken in - Yukine needs that, because he’s just a child with massive abandonment issues, carrying tons of subconscious baggage from his previous life, who was abandoned by everyone. That sense of purpose and belonging is extremely important to someone like him, hence why his attitude changes so fast.
I can’t find it at the moment b/c it’s been a minute since I reread the manga but there is a certain moment where Yukine states that he knows why he exists now, again showing that Yato has given his existence a sense of purpose.
Yukine himself even acknowledges this in Chapter 78, a whopping 67 chapters later, when Nora talks about taking on all of these names but still feeling like she is missing something.
And that’s where we also see a parallel between Yukine and Nora on this exact topic - Hiyori mused several chapters ago that because Father gave her a name, he also gave a purpose to her existence, hence why she keeps coming back to him and why he is so important to her, despite treating her horribly.