[To a small degree Zero's was a different case. It was a "what if" scenario, just as much as Yuuki's was. However there's one thing he can't help but feel a different sort of heartache over. Ichiru. Light-hearted, confident, healthy, by his side. . . everything that Zero had always wanted when they were growing up. And so in the here and now, yes, he does mourn the fact.
On the other hand, however, it's so far from the truth. Having experienced what he did (still) reinforces the fact that Zero would not have changed anything about his past, save maybe for the way he had pushed Yuuki away on that night. Never again. Even through tragedy had he been given something so precious; something he'd never give up. She's always been his anchor, his light.
Right now he's hurting more for her. Even within his own guilt for having forgotten Ai, as well.
He moves to slip and arm around her shoulders, a hand resting upon her head.]
No. You're not.
[Quiet. He coaxes her a little to lean against him.]
It was never your fault. Things are going to be okay, now.