There's a moment, a space of a few seconds between when Draco tells them his full name and when he's pointed his wand at Lucius's face. It's enough time for his own wand to find it's way to his hand, and were he sober, enough time to block the incantation. He spent time as an Obliviator, after all, he certainly knows how to avoid the spell. But he's not sober, he's far from sober.

He uses that time to think, then speak, instead of defend, and the only thought that manages to form is that he needs to figure out what exactly occurs in this future. "Your son," he begins, and really, that must be the last straw because then he thinks nothing at all.

Obliviate is, for several long seconds, like being trapped in a bubble as memories are locked away to where they can't be accessed. He's still there, but he's not really aware. He's frozen in place as Bella tries to talk Draco down. But if he weren't, he'd likely tell her it isn't worth the effort. That they'd been pushing in a way that a Muggle wouldn't have realized but a Malfoy certainly would; that in Draco's position Lucius would be doing far worse than Obliviate.