I think any accurate portrayal of grooming has to be both subtle and repulsive. It's easier to write/read about the monster lurking around dark corners, with abuse centred on outright violence, threats of violence, and a victim who is physically unable to leave (eg the first part of the book Room). None of that's pleasant to contemplate, but it taps into simpler-to-resolve conflict patterns: there's a goodie and a baddie and the goodie must take steps a, b and c to escape.
What Lolita portrays is a much more disquieting (as opposed to in-your-face terrifying) story of a trusted and well-presenting adult insinuating himself into a child's life and taking advantage whenever the situation permits it: at first occasionally, then constantly. This would be a very sad and horrible read if told in the third person, or by Lo herself, but the fact that 95% of the book is narrated by the abuser is what makes the whole thing so brilliant (but of course still sad and horrible).
Humbert is an eloquent narrator who tries to justify his behaviour in a million ways: citing historical context, analysing his own childhood, elevating lust into a great mythic romance of fate and reincarnation, portraying himself as the love-struck victim and Lo as the goddess/temptress, claiming that the whole affair is of her own volition.
It's all bullshit and, at very rare moments, he acknowledges it. We see that his behaviour is disgusting, that he is sane enough to see the damage he's done, and that he wrecks this child's life anyway. I don't buy the final 'she's not a child any more but I love her anyway and always shall' excuse. IMO, that's just Humbert's final attempt at justification. They're not star-crossed lovers. He barely knows this girl. But: UNREQUITED LOVE, so that's okay, he's suffered too.
Taking all his justifications at face value just proves how scarily persuasive people can be. Poor Humbert is the hapless suitor, the dazed foreign academic, the nice guy who's done everything he can to avoid hurting children. Lo must share equal (if not more) blame, for being sexually active and 'wayward'. In the end, theirs is a great, eternal and inexplicable love -- that's what Humbert implies. That's not what the book itself claims. If anything, the fact that people can and do fall for this reverse-victim/respectable-man/immortal love manipulation from an abuser is exactly what the book demonstrates.