Apparently the Blackstaff cannot ward off that kind of attack, Eb did end up in surgery at the end of Battlefield. However it might protect the wielder of the Eye from it's hate, just as it protects it's wielder from black magic.
That's fair. I think he was in surgery because of the roundhouse kick that Ethniu gave him. He did survive Gungnir's attack, which I thought was quite impressive. But I do agree I think a shot from the Eye might have been too much.
I don't think the Blackstaff can tank a direct hit from the eye without someone incredibly logical holding it. My point was more referring to the hate radiation it would give off were Harry to try and use the eye. I don't know how else Harry would be able to use it, though maybe Harry has it to take it off the field as he's the one person you can trust not to use it.
I wonder if Balor really didn't like the Outsiders for some reason or if he was corrupted and that's why the Twyleth Teg (?) had to take him down.
And if Jim is putting in Kaiju we are absolutely getting a Mecha of some form.
My reading of how Mab survived (and it's mostly just me speculating here) is that whatever Mab's defensive spell(s) were, it at least partially allowed her to redirect the energy somewhere else. Obviously not all of it as she still got fried. But she literally worked out the math of how much energy would be fired at her and how much she would need to block/redirect in order for her body not to be destroyed, and then figured out an appropriate defence. She didn't need to just have faith her shield would work, or just hope that it would do the trick, she literally had worked it out and held onto that reasoning. Which is how her logic overcame irrational hate and rage. If Ethniu had been a bit more logical, she might have guessed Mab would prepare such a counter. In fact, she probably should have just kicked her again. Clearly in physical combat she had no equal on the field. And I'd say in sorcerous might (perhaps not skill) and raw connection to magic, she also was unequalled. She might have actually killed Mab if she had just fought her physically or with her own magic. I mean, she even had proof Mab was vulnerable to that because of how well her kick worked in Peace Talks. Instead she opted for maximum carnage, as Mab likely guessed she would, and Mab had a very excellent counter for it. As Marcone pointed out, Ethniu's predictability is what made her vulnerable.
I agree, he might not be "tainted" by the hate in that sense. His soul might be less affected. Or his mind for that matter. But the hate was given form in a sort of heat energy that generally destroyed everything it touched. I don't think even the Blackstaff would save you from that.
The only way I can see Harry being able to use the Eye would be as a big battery for a spell. I doubt he will ever just be able to point it like a gun at anyone. Could help him in a fight against Demonreach though if they have a falling out. I agree though that Harry mostly just wants to remove it from the field, and Marcone let him because he doesn't want that weapon in the hands of enemies that could use it like Ethniu, but also probably will use it as part of a case to undermine Dresden and say "Dresden is the bad guy, not me" while pointing Harry's worst enemies in his direction.
As for Balor, I don't think he hates Outsiders. What's your theory there? My guess is he actually is connected to the Outsiders, he is somehow on their team. Maybe they even helped him with his Eye. Assuming that the Outsiders are not merely the forgotten/fallen gods etc. (which sometimes I think might be true, and sometimes I think it's probably not). Balor might well have been corrupted...but then again the Fomor in Irish mythology/legend were always pretty much the bad guys. They might have just started out as evil.
I suspect the Mecha will be human (not Wizard) created and controlled should they appear, but we'll see.