The sloppiness of this season can be redeemed if they actually use it to fuel the plot. Dexter was careless even within the unlikely confines of the show's premise. All these things he did wrong or badly this year would be the perfect fuel for a reveal next season.
Having Lumen 'cured' was lame, no real argument against that. Having her remain damaged but refusing to go down that path any further, trying to drag Dexter with her out of the dark as he deals with coming under increased suspicion from all his fuckups this year and the slow boil of pressure from his 'dark passenger'. . . that would be a good story, especially if it reveals him in the end and sets up a final blowout season. .