Nothing is justifiable or non-justifiable per se.
Age of consent is just a post-Victorian idea in western culture, anyway. Not saying it was better before, not saying it's bettter now. I can form opinions without someone preaching me and I don't have a need to preach mine to others.
As for Humbert,.we can't even know how much of his narration is veritable, he himself says he can't be sure or bothered by details that do not change the whole picture (and it's emphasised on almost every fucking page - his SUBJECTVE PICTURE) after all the time that passed from the start of the story to the point where he writes it in jail.
She was Lo, plain Lo, in the morning, standing four feet ten in one sock. She was Lola in slacks. She was Dolly at school. She was Dolores on the dotted line.
But in my arms, she was always Lolita.