Fairy Tale AU + Fake Married, your choice of characters or if you would rather a specification Edith /or& Laura!
OKAY! After a good night's sleep and a lot of turning things over mentally, considering and discarding options, my brain has finally settled upon the following:
1) This would take place in roughly the AU equivalent of seasons 2/3. The timeline would not match exactly, but meh, it's an AU.
2) Since DA already has the aristocracy, everyone will maintain their canon titles.
3) While I love Edith and Laura, you gave my brain a choice, which means my brain went running elsewhere. ....actually, my brain went gallivanting off in five different directions before I finally go it to choose one.... It settled on Anna/Bates with Fake!Anna/Molesley.
Larry Grey, future Baron Merton, desperately wanted to marry Lady Sybil. When she spurned his affections and turned instead to the coachman - the Irish coachman, of all things! - he, being a vindictive little snit, decided to make her feel the suffering of being passed over for a servant (because this was the sort of thing Lady Sybil would care about, clearly, obviously....okay, maybe not) and started obviously courting Anna Smith.
Anna's fiancee, John Bates, had been imprisoned for a crime he didn't commit. Larry used Mr. Bates's absence as an excuse to pursue his overtures. When Anna complained to her employer, Lady Mary (after saying several cutting things about Larry's intimate anatomy) suggested that they enlist Lady Violet's aid, since Lady Violet was the last of the Crawleys to inherit magic from their illustrious fairy ancestors. After much debate, the women decided their best chance was to find a willing man and cast a glamour on him to make him look like Mr. Bates.
The hunt was on! The man in question needed to be kind, firmly on their side, but able to be absent from the town for an indefinite period of time. They finally settled on Joseph Molesley (somewhat reluctantly since Anna knew he'd been interested in her at one point and didn't want to rub salt in the wound. She was gallantly informed that Molesley was past that and even if he hadn't been, this was more important).
Eventually, though, Mr. Molesley “took a job elsewhere” (the job would fall through and he’d return to Downton when Mr. Bates was released), the glamour was cast, a fake ceremony held (they figured they could do a real one when Mr. Bates was back), and everyone gave a sigh of relief when Larry stormed off. It was generally agreed, however, that they should keep up the charade until the real Mr. Bates returned, just to insure they stayed Larry-less.
Of course, no one stopped to consider that one of the younger members of the staff had enough fairy blood to see through glamours and didn’t like Mr. Bates...