Others here have told me that the ghost would have been Stoddard's (which makes more sense, even to Jeb!), except that Dennis Patrick was unavailable. That's why it made no sense. Bradford was shoehorned in for no good reason. I'll guess that the writers weren't responsible, but instead DC and/or the fact that it was too late to rewrite the plot after D Patrick bailed out.
All this implies that Jeb goes through the accelerated Leviathan growth process every century or two, whenever they want to try taking over the Earth again. It had seemed to me that "Jeb" was a new being in 1970, since he's so unfamiliar with humans and life as a human, but what the hell. Apparently they went through all this with the Leviathans in 1796/7, after all the events we saw in 1795/6, and they failed. Somehow Bradford and VW were involved.
Does this half-explain why the cairn and Merle Oberon (sic) were there at Collinwood in 1795/6? They'd just failed in that time, and needed to try in a new one? No, since this is concurrent with Barnabas trying to make Josette a vampire, and it looks as if any Leviathan activity is later.... but maybe not. The Leviathan thing could have been brewing for awhile in 1795/6 without the onscreen DS characters being aware of it, or affected by it, till later on after Barnabas is chained.