I feel like Nico got really angsty and angry because Percy fell into Tartarus. I feel like he was really angry that all these years he never told Percy and suddenly, Percy might die, and Nico would've never told him. I think that's why he was so angry in this book. Also the abbreviation is BOO HOO. Really Uncle Rick? Really?
I really think that Magnus's father is Lucifer or Satan himself and that's why he doesn't like to talk about it. I also think that somehow they cure Sebastian because in a snippet, Clary is talking about how her brother has green eyes, and he obviously has black eyes as Cassie clearly states many many times.