I'm currently working with an exec who worked on this, and he told me they couldn't recast because it took Elgort eight months to learn Japanese, and they obviously couldn't wait *another* eight months for a new actor learn Japanese + the time/resources for reshoots. It sounds like the project was potentially in jeopardy of not even seeing the light of day, when they decided to take their cue from Spielberg. If Spielberg released West Side Story with Elgort, they'd do the same. So when Spielberg moved forward, so did they.
That's a long way of saying, at this point it still might not even be worth it for HBO Max. Having another actor have to spend eight months learning Japanese is probably a headache everyone wants to avoid.
As for the allegations themselves, yeah, nothing was ever proven or tried, though I'm pretty sure there were screenshots of him soliciting the accuser over DM on Instagram, when she was 17 years old, and he was like 23 or something. He claims he didn't know she was 17, and that the assault didn't happen or that it was consensual or something, but I think the screenshots were enough to scare everyone away, at least for the time being.