I think I'd rather trust Toga on what she thinks about her quirk than Curious and Shigaraki doesn't act like this is anything new. I mean, why would Shiggy charge them all if he didn't expect this to happen? What is interesting is his chat with himself about seeing things that are not there and his little flashback, which stinks of All for One fuckery.
Toga didn't really think of any sort of explanation, she was just surprised. That implies it's something new to her, which would still work in the context of Curious' theory. As of now, my belief is that heightened emotions can unlock "power ups," in a manner similar to adrenaline rushes. The latter would allow a mother to lift a car to save her children, and in the case of quirks, heightened emotions allow them to push their quirks further to do things they normally wouldn't be able to do. Then, over time, I expect Toga to be able to use other people's quirks on a regular basis.
Essentially, I believe heightened emotions would allow one to skip the gruelling training (like what 1-A and 1-B have been going through) and use "power ups." It's like a momentary burst of power that gives them a taste of what they could do in the future. It's much like Deku accidentally tapping into Black Whip only to not be able to use it regularly.
You're right about the flashbacks. There's definitely more to them... I'm getting more suspicious of All For One and more interested in Shigaraki's past.
Speaking of that flashback, it seems to imply that Shigaraki wanted to be a hero but his father did not approve of it. Is the latter new info? I know Nana left her family (or neglected) due to her work as a hero, so I'm not surprised her son doesn't approve of his own son becoming one.
Yes, as it always has.
Quirks=personality=emotion
Not exactly, though? Emotions are a part of one's personality in they can describe each other, but a character's emotional state hasn't affected their quirks before. And I'm specifically referring to power ups and new abilities. Prior to Black Whip, we've only seen new abilities as a result of physical training.