I think that little metaphor was more directed at A than at the MC. The green and red leaves that bite into the brickwork, holding it up but destroying it, are supposed to be A’s past and ways of dealing with trauma. A uses their past as a reason to keep people at arm’s length (save UB), and as a shield, in some respects, because they don’t want to risk losing anyone (or causing the death of someone they love) again.
So yeah, it’s not about the MC being destroyed. It’s about A destroying themselves with the very thing that has kept them going for so long.
I think it could have worked, even without the rivalmance to lovers. Imagine this…
You are a supernatural creature, one who is very in tune with magical things, able to recognize that which remains unseen to others, especially human. You know soulmates exist, know that such a connection can be recognized immediately (or, possibly, slowly recognized if you aren’t in tune to such things). Yet, you’ve never believed in it for you. You’ve seen it with others, maybe even recognize it on your own team (in which case A would be able to “see” that connection between the MC and their LI on other paths), but never for you. You’ve been alive too long, seen too much. Been through too much loss, some you feel you’ve caused.
In walks the MC and it hits you like a fucking avalanche. You know when you lay eyes on them that they are the one. You feel it in your bones. In your heart. In your soul, which you thought damned for eternity for all you’ve done. Worse, it’s a human, one who probably doesn’t return your feelings, one who doesn’t recognize that connection that is already digging its claws into your very essence.
You are, for the first time since you were turned into a supernatural, afraid. Given A’s personality, that fear would manifest as anger, so the constant asshole behavior in b1 would be understandable. After all, the MC will soon be out of A’s life, so it’s best to make damned sure to keep them away. If you don’t, then you won’t be able to resist. But then the MC is made liaison.
And this is where it would have to shift gears. In b2, it can’t be the fear displayed as anger anymore. No more hot/cold or combative behavior. Here, it would have to be obvious fear. A running away after a touch or getting too close would be the only protection they have now. But even that has to evolve…
So in b3, you’re past the rivalmance, past the days where A tucks tail and runs. And here is where you need honesty. A’s explanation about their family. A letting the MC into their life with the wash-the-car date. The walls start to come down, and if they start to go back up again, the MC can put a stop to it and, with the right choices, force them down further. Keep the kiss at the end, but without the “after this it’s over”.
Have the love admission and the kiss, with A flat out telling the MC about the soulmate thing. About the magic between them. About their own fear of it, because the MC is human and A can’t bear the thought of losing them, where A says, still clearly afraid, but full of emotion and need for the MC, “And I do not know where to go with this now. With us…” So the MC can tell them they’ll figure it out… together. It gives more hope, continues progression in the romance, and keeps A in a place where they still have shit to work through.
I think that would’ve worked.