I know so many missing pieces of information and yet we are in the final act. I don't think Crane is part of the cult otherwise the kid probably would have delivered the note. And it seemed like the sorcerous sundaries assumed we stole the dagger for Crane and wanted to sell it. Why steal one if you are part of the cult?
In regard to the bard I suspect the beard disguise was unrelated to the cult. To meet a secret lover or skip town after being discovered or some other activity where there was a motive for someone other than the dragonborn to murder him.
