I think the police would of course have been all over AC as it does seem the most likely scenario that he would have something to do with it. But the fact they have looked into him and don't think he's a suspect says that they must have something really concrete to rule him out. Maybe they have looked through all his phone records and messages and it proves his relationship with Leah ended quite some time ago and he's had no contact with her since. Maybe he was out of area during the disappearance and they can prove it.
My theory is that Leah had entered into a new relationship with someone else and no one else knew about it because it was so new but also that potentially there was some need for secrecy about it, maybe another older man or something similar. Either this person was a predator who was preying on her from the start and planned her disappearance or something has happened in the heat of the moment and they've simply been unusually fortunate getting away with it for this long.
Leah sounds like someone quite shy,naive and young at heart. When they did a tour of her room on This Morning you saw that she loved to read fantasy and adventure stories - twilight stuck out to me particularly as it's a story about a girl who is quiet and reclusive (like Leah) getting into this whirlwind romantic dangerous adventure with this much older experienced guy and I feel like Leah loved this sort of thing as she was longing for someone to bring exciting romance into her life and unfortunately I think that is what she has found but it has taken a dark turn.
My other thought based on the distressed girl by the lake sighting and police saying (either the This Morning clip or crimestoppers, can't remember which) that she'd been anxious or worried about something recently, was that following things going south with whatever relationship, she was feeling unable to cope and took her own life. But of course unless she's gone somewhere very remote for this it seems very unlikely given no body after 7 months.
(Disclaimer - not hating on or passing judgement on twilight series at all, merely an observation into Leah's potential mindset and personality through her choice of reading! I myself loved a bit of vampire/horror/fantasy books as a teen and it definitely said something about my personality so feel like I have a bit of insight)