Osamu not meeting Yuma is an interesting alternate history. Yuma was aiming to talk to border higher up from the start, and without having to protect Osamu from trion soldiers he would draw less attention to himself, so it might be more peaceful than it was. Most likely they'd still fight and Yuma would eventually go down to the away mission squads. Probably. With replica his black trigger can create a fucking Rabbit which is meant to cost the equivalent of an entire trion army, so who knows what he could do. Maybe he'd summon a fleet of Illgar, crash them into HQ and kill everyone. Maybe he'd just Gate himself off to another dimension.

With Chika the question is whether her side effects would hide her from Aftokrator's scanners. They probably can, Afto was surprised when they got her readings after she shot, so Chika gets evacuated like a civilian and no-one ever finds out she's got God-tier trion. Without her blowing a hole in the base it's never reinforced, so it gets seriously damaged.

Osamu just dies to the Marmod. He'd try to stop it even if he'd never got into Border. His tombstone reads "Here lies Four-eyes. He never scored."