Responses
I also think that he went back in time and has the memory of an adult. His mindset, behaviour and knowlege, skills, reading. He might even have a hint, that she isn't his true aunt. Maybe that was a part of his original wish? To have a family, to have a loving mother? And whatever granted him thos wish and sent him back in time also switched his aunt for someone capable of caring for him?
I think the grown up Luca was sad because he didn't have anything he could care about so maybe he wished to go back in time and fix the things he could. and maybe his wish came true be he returned to his past. That's why he behaves like an adult. A very good example of this is when they were traveling in the train. Luca showed no interest in the train though he never saw one and he even attacked the assassin like he knew how to use a knife which is very unlikely for a little kid like him.