A Fullborn can tap enough strength to start breaking it
One, a Fullborn will no matter be faster than the Radiant, two, Fullborn will resist all soul casting themself, and three, even you turn the air into metal, he doesn't need to breathe and can push it off his head and heal from it.
And? What's the Radiant going to do? You can't surprise someone who can speed up their thoughts and react in a split second not to mention move to them in a split second.
Not really. you can kill a Radiant with Stormlight if you decapitate them or crush their skull, which a Fullborn can definitely do, while Fullborn's healing will make them survive pretty much anything. It's definitely an added advantage, but even if he didn't, he could still kill the Radiant easily.