He raises his arm to block it with his shield, a quick calc give me some bad results though. I found the distance between them is 11.37 cm, but even assuming the explosion lost 50% of it's speed, I'm getting results that are too high. Around Mach 90 or HHS+ Speed, which is an outlier since Wounded All Might is Mach 26.
Even assuming the explosion lost 90% of it's speed I get Mach 18 results, which is still an outlier so.
So yeah it's no good. I'm pretty sure 8% Izuku didn't react but predicted a casual Mirio, so I'm not sure if that means he'll downscale.
Also no on the Rappa thing, Knuckleduster said he couldn't find an opening to attack, but he could parry Rappa's punches just fine and even fake being knocked out by him. So Rappa just scales to Overclock's speed, he doesn't scale above. He may have gotten faster over the years, but by an unknown amount.