B) It serves no purpose plot-wise. There was literally no need to OL to suddenly, instantly lose consciousness in the middle of the fight. It wasn't that Nabu was targeting him (he wasn't), it wasn't highlighting some weakness that OL didn't know he had (he isn't weak to magic anymore, and hasn't been for a while now. But suddenly, he's rendered instantly unconscious by a mana explosion minor enough to not kill the guy whose face was literally in front of it? Or injure anyone else? Through all of his multi-layered defenses, including his spelleaters, which are extremely effective at flat-out absorbing magic? How was he even rendered unconscious instantly, when nothing else ever has when he's had any kind of defenses up at all? If the mana-explosion was really that tremendous, why did it simply speed up the mana-absorbtion process up even faster? What actually knocked OL out? It can't have been the blast wave, since he's protected against that very thoroughly, and no one else was affected. Was it the massive amounts of mana? But what about the defenses and spelleaters, and how on Earth was it instantaneous? There should have been at least a second for the spelleaters trying to handle the strain before failing. And, again, no one else was affected, even the guy who was literally trying to kill himself with the explosion and was right next to the blast's origin.)
C) It does not make things more realistic. If anything, it's jarring at how unrealistic it is. This isn't some weakness OL has, or some new vulnerability. It's something he's dealt with numerous times before suddenly affecting him in a way that's completely different from all those other times, despite him being better prepared for it.