I think there's more of a connection there than just the fact that Diabolos might have dispatched Nicholas to save her. It's no coincidence that she introduced herself to Dr. Lang as "Miss Blair" (and it's always thrilled me that the writers remembered that little detail, because as we've all seen, their attention to continuity detail becomes sketchy at best later on). But Nicholas mentions a "bargain" between the two of them, and he was the one who sold the portrait of Angelique to the man in the antique shop (I think; suddenly I'm very fuzzy about that particular detail, but I'm sure that Stokes mentions it sometime soon, and Nicholas pretends not to know what he's talking about).
As far as Nicholas and his prior incarnations -- I used to think that Evan Hanley and Charles Dawson were earlier versions of Nicholas, but I've scrapped that idea. Dawson, maybe (but I still say no), but definitely not Evan. Nicholas was always so cool and urbane (even when he was snarling at people), and Evan just ... wasn't. I believe he's always referred to as "an inept Satanist", and Nicholas is anything but inept. Though come to think of it, neither of his big schemes came to fruition, did they ...
Nicky