She did die in 1795, and she is not a ghost.
In one of yesterday's episodes Julia told Vicki that if Cassandra is Angelique, she's not a ghost -- she's flesh and blood.
I believe Angelique physically resurrects, though she has the ability to slightly alter her appearance (e.g., her hair).*
At a later point in the series, Angelique taunts the imprisoned Victoria with the fact that she is flesh and blood. The scene parodies one of the resurrection scenes of Jesus recounted in the gospels, right down to the specific language.
We don't know just when she resurrected herself (through Satan's power) as Cassandra, but it may have been fairly recently, allowing enough time for her to scope out the current Collins family before springing her plan ...
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*And physical mannerisms -- see my "Kitty Cassandra" post. Altering one's physical mannerisms is NOT an easy thing for anyone (other than a trained actor) to do.