The definition of “plot device” can be tenuous.
Was Lacey there to throw a spanner in the works? Sure she was.
But she also served to influence Rumple’s character in such a way that he reached the point we saw him at in the finale…that heartbreaking moment when he resigned himself to death because he had no one left to truly live for, which made his moments of clarity all the more rewarding.
Lacey also assisted in Regina’s journey, because she was an example of how low she’d sunk, that she was resorting to messing with Rumple like that. It was one of several factors in her hitting rock bottom. And she wouldn’t have been able to have her turning point if she hadn’t have reached rock bottom.
For Belle herself, she’s gonna have memories of being Lacey….memories of being pro-magic…which she has to be if she’s gonna cast the spell that Rumple left with her. Would old school Belle have agreed to cast that spell, knowing how magic can corrupt, and had corrupted Rumple? Maybe not. But her Lacey-founded attraction to Rumple’s magic could now serve her well, not just with this spell, but also as some have been speculating, if they bring the enchanted mirror into the storyline in the future.