There is no way to prove it but I just don't see any predetermination here.
It would be trash writing if the climax didn't follow logically from the events that led up to the climax. And conversely, it's actually good writing if the author has the characters behave in a consistent manner such that the climax is kinda inevitable. That shows forethought and planning.
He clearly shows he cares a lot.
Caring for someone you don't trust is kinda a hallmark of toxic relationships...which I don't want to argue w/ you about whether the Aya/Yuu ship is toxic, that's an agree to strongly disagree. I just want to point out that caring and mistrust are not mutually exclusive by any stretch.
So again, imo he is not the type to fall and stay happy with a person he distrusts.
They're in the honeymoon phase of their relationship - it's been like 2 or 3 months, they're screwing all the time, and Aya actively deflects/avoids/redirects just about everything where trust could be an issue (on either side). They don't meet each others friends or family, they don't talk about school all that much or really anything else personal, and they don't really hang out in ways that require showing up for the other's interests.
Like "hey move in w/ me" is the first real test of whether he trusts her b/c it's the first time she's really asked him to potentially make a sacrifice/do something hard. Eta: you argued it was about commitment, but like why would he not be committed to the girl he, in your words, loves? And if he was committed, why not give her an answer that reassures her that he's committe? (even if moving in is ridiculous cause they're teens)
So it's not just about giving up after the first failure or being cautious
But like there wasn't a first failure b/c he decided Hikari's answer w/o talking to her. Yeah I agree w/ you that he may very well have been right, but like the brave thing would have been to face the potential emotional damage of an unequivocal rejection. Not verifying w/ her leaves it as a forever "maybe she'll say yes".
ETA: which again to emphasize, this is all part of the discussion of why some folks find Yuu unappealing as endgame for Hikari, I don't think any of it makes him all that terrible and it's all fairly realistic.