Responses
No he realized she wasn’t a kid because of the comment she made about them meeting only in the rain. He knew she wasn’t the princess… he had unconditionally had his suspicions already and that comment confirmed it for him.
Maybe I'm crazy but I don’t find it strange him proposing. Lusha finally spoke about her past life to everyone, so they are aware her mental age isn’t that of an actual child. Feyran never confessed to the princess because 1, he wasn’t fully aware of his feelings until she died. And 2, even if she hadn’t passed, he couldn’t really pursue her because she was a human and ages much faster than a divine being would. I understand that in her current life she was just born but it’s not as if she’s going to procreate with him and currently even he’s aware she’s a kid and is only proposing. Feyran is going to wait for her. That’s just my thoughts though ╮(￣▽￣)╭