Here are some theories:
Nico's neighbor Pietro, the one that went to Africa and came "back from Mussolini's African campaign in one piece, but after shelling Ethiopian civilians with mustard gas, his mind was never the same." might have been a love interest of Nico. Let's go through the facts we know that then we think he knew he was gay. He seemed to be close to Pietro and when he came back not mentally stable he seemed to really care. Maybe it will come up in the Solangelo book.
Rachel's hairbrush is magic since it hit a literal titan's eye I think there must be some kind of magic or something special about it. If Hephaestus' godliness rubbed off on the tissue/Athena's dress, why can't Kronos' titanliness rub off on her hairbrush?
When Percy and Jason were controlled by the Eidolons they regained consciousness. If they wanted to kill each other and Piper stopped them, they could easily just stab her or something it's not like the Eidolons are such gentlemen or anything.
That's all, for now, more might come soon so be sure to check it out.