"History is always being rewritten."-- Barnabas Collins
I always took for granted that Barnabas made Roxanne a vampire, in either time line. Maybe 1840 Barnabas had a brief stint out of the box thanks to Angelique on one of her yearly visits, in 1840, 1841, 1842...? I imagine she was tempted every single time. Ang certainly would have been around each anniversary, but probably would not have cursed Roxanne directly, since she'd have no apparent reason to, but who knows?
As I notice more and more holes and problems in all the storylines, 1840's errors seem less important. 1968 was a grab-bag of whatever they could think of thrown together, with rules going out the window, so compared to that, 1840 seems grounded and reasonable. 1840 feels much more like conscientious, solid, serious DS than 1968 does, to me, despite many flaws.
There's no logical reason in particular for ghosts to look like their former bodies. But they do, so I'd guess that after living for awhile as Gerard master of Collinwood, that's Judah's body-- so that's what Judah's ghost looks like. The body at the time of death may determine what image the ghost has. (The question for me is, what happens to the soul of the real Gerard?)
Another possibility is that ghosts can choose their appearance, and Judah was just as dead-set on being master of Collinwood in death as he was in life, and that means being Gerard. Gerard has a scarier glower than Judah too.