Sometimes letters are not sent or sent with a lot of delay. But DM on the sponsor side is SA.
Regarding visits, I don't think they do anymore. I heard that in the past, but even then, it was very rare. In a relationship interview they can spot quite rapidly who is in a genuine relationship and who's not.
But I'm not sure this is what will happen to you neither. But when the file gets transferred to the local office before AIP, this is normally a sign there is something they are not convinced about.