Corban was knocked several meters by the explosion regardless since its in my post. As for the barriers, I would say he isn't 'deflecting' the blast. He repositioned it away from vitals. Your second theory is the one im willing to go with (barriers summoned, but not at maximum power). I'm willing to accept some second degree burns(third causes incredibly extensive damage, not a 'mild impediment', unless we'd agree the surface area of them was relatively small), suffering a few holes in his robes where the damage was dealt. 

Corban is not very good at self-healing. The typical 'heal' spells fall under the conjuration class, which he isn't very good in. However he has other methods of damage control and mitigation, just not outright healing.