I think to help make your claim more believable, you could claim that you can't convert other people into masons (Unlike normal masons whom would convert someone each night), but I try and make the cult look somewhat weaker than a normal cult. I was thinking maybe making Duffie's role look like {Cultist - Meets with other cultists every odd night to convert one person. Wins if they equal or outnumber the rest of the town}, so that we don't need to bother with the cult leader business, the cult looks somewhat weaker (but will still cause panic when they can't be found), and you can make it so your roleclaim doesn't have to convert someone into becoming a mason each night.
Also, the plan works even better since you will appear inno on reports, and any trackers/watchers would see you killing Duffie tonight (since you're Godfather).