Given that I seriously doubt that Maruto is unaware of probably the most commonly used trope in romance (including manga), I seriously doubt he unintentionally wrote the peak miscommunication scenario of "despite trying their hardest to hide it from Hikari, the information gets disclosed to Hikari in the most painful way possible".
I guess we should stop this discussion about inevitability because it doesn't seem to go anywhere. We just can't convince each other, so let's agree to disagree once again.
Not putting his all into the exam, which he discussed with Hikari when he owned up to it - that's a classic is easier to justify failure as a well I didn't try all that hard.
He never mentioned "not putting his all" into the exam to Hikari. Instead, he told Yami in chapter 23 that he should've worked harder, that he regrets so much that he didn't study even harder. And how is that chickening out? He literally put lots of effort trying to get into the school that was out of his league just for Hikari, failed and still didn't say that it was pointless, that he should've worked himself to the bone. That's almost the opposite of chickening out.
Not telling Hikari he was trying for the same school, despite them being close b/c he didn't want her to know if he failed.
That would count as chickening out if he was still pursuing romantic relationship with her. Otherwise there's no point.
Not being a firm no on going to the school festival if he really didn't want to tell her what's up
I'd rather call it not having enough resolve, but sure, let's count this as an example.
The closest he gets to challenging Aya on anything is that closing convo where he's like "wtf, I'm falling for you" but he doesn't push it at any further than that even though that's a bonkers thing to say to your BF. And like he doesn't think it's her usual teasing* given that he responds with a "wait what no?!"
He most definitely thinks it's her usual teasing (which it is), hence the comically flustered and frustrated reaction.
You really have a very different interpretation of their relationship from mine. For me it's pretty clear that they both enjoyed their teasing senpai and earnest kouhai dynamic. That's how it all started, even in chapter 22 when Yuu is down in the dumps and Yami teases him about his failure, he ends up saying she helped him cheer up, meaning he ended up enjoying their communication. There was definitely miscommunication later from Yami's side (in how she kept her deep affection for him to herself, playing the above mentioned role instead), but imo it's far from the toxic distrustful relationship you seem to have in mind.